Refactor and clean up IndentationRule

### What's done:

 * See #1448.

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/files/IndentationAmount.kt

@@ -0,0 +1,59 @@
+package org.cqfn.diktat.ruleset.rules.chapter3.files
+
+import org.cqfn.diktat.ruleset.utils.indentation.IndentationConfig
+
+/**
+ * Encapsulates the change in the indentation level.
+ */
+@Suppress("WRONG_DECLARATIONS_ORDER")
+internal enum class IndentationAmount {
+    /**
+     * The indent should be preserved at the current level.
+     */
+    NONE,
+
+    /**
+     * The indent should be increased or decreased by 1 (regular single indent).
+     */
+    SINGLE,
+
+    /**
+     * Extended, or _continuation_ indent. Applicable when any of
+     * [`extendedIndent*`][IndentationConfig] flags is **on**.
+     */
+    EXTENDED,
+    ;
+
+    /**
+     * @return the indentation level. To get the actual indentation (the amount
+     *   of space characters), the value needs to be multiplied by
+     *   [IndentationConfig.indentationSize].
+     * @see IndentationConfig.indentationSize
+     */
+    fun level(): Int =
+        ordinal
+
+    /**
+     * @return whether this amount represents the change in the indentation
+     *   level, i.e. whether the element should be indented or un-indented.
+     */
+    fun isNonZero(): Boolean =
+        level() > 0
+
+    companion object {
+        /**
+         * A convenience factory method.
+         *
+         * @param extendedIndent the actual value of ony of the `extendedIndent*`
+         *   flags.
+         * @return the corresponding indentation amount, either [SINGLE] or
+         *   [EXTENDED].
+         */
+        @JvmStatic
+        fun valueOf(extendedIndent: Boolean): IndentationAmount =
+            when {
+                extendedIndent -> EXTENDED
+                else -> SINGLE
+            }
+    }
+}

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/files/IndentationAware.kt

@@ -0,0 +1,11 @@
+package org.cqfn.diktat.ruleset.rules.chapter3.files
+
+/**
+ * A contract for types which encapsulate the indentation level.
+ */
+internal interface IndentationAware {
+    /**
+     * @return the indentation (the amount of space characters) of this element.
+     */
+    val indentation: Int
+}

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/files/IndentationConfigAware.kt

@@ -0,0 +1,76 @@
+package org.cqfn.diktat.ruleset.rules.chapter3.files
+
+import org.cqfn.diktat.ruleset.utils.indentation.IndentationConfig
+
+/**
+ * Higher-level abstractions on top of the [indentation size][IndentationConfig.indentationSize].
+ */
+internal interface IndentationConfigAware {
+    /**
+     * The configuration this instance encapsulates.
+     */
+    val configuration: IndentationConfig
+
+    /**
+     * Increases the indentation level by [level] * [IndentationConfig.indentationSize].
+     *
+     * This extension doesn't modify the receiver.
+     *
+     * @receiver the previous indentation level (in space characters), not
+     *   modified by the function call.
+     * @param level the indentation level, 1 by default.
+     * @return the new indentation level.
+     * @see unindent
+     * @see IndentationConfig.indentationSize
+     */
+    fun Int.indent(level: Int = 1): Int =
+        this + level * configuration.indentationSize
+
+    /**
+     * Decreases the indentation level by [level] * [IndentationConfig.indentationSize].
+     *
+     * This extension doesn't modify the receiver.
+     *
+     * @receiver the previous indentation level (in space characters), not
+     *   modified by the function call.
+     * @param level the indentation level, 1 by default.
+     * @return the new indentation level.
+     * @see indent
+     * @see IndentationConfig.indentationSize
+     */
+    fun Int.unindent(level: Int = 1): Int =
+        indent(-level)
+
+    /**
+     * @receiver the previous indentation level (in space characters), not
+     *   modified by the function call.
+     * @param amount the indentation amount.
+     * @return the new (increased) indentation level.
+     * @see minus
+     */
+    operator fun Int.plus(amount: IndentationAmount): Int =
+        indent(level = amount.level())
+
+    /**
+     * @receiver the previous indentation level (in space characters), not
+     *   modified by the function call.
+     * @param amount the indentation amount.
+     * @return the new (decreased) indentation level.
+     * @see plus
+     */
+    operator fun Int.minus(amount: IndentationAmount): Int =
+        unindent(level = amount.level())
+
+    companion object Factory {
+        /**
+         * Creates a new instance.
+         *
+         * @param configuration the configuration this instance will wrap.
+         * @return the newly created instance.
+         */
+        operator fun invoke(configuration: IndentationConfig): IndentationConfigAware =
+            object : IndentationConfigAware {
+                override val configuration = configuration
+            }
+    }
+}