refactor(ui): new MainViewModel

ViewModel in a lifecycle component that can help share necessary live data among Activities.

app/src/main/java/com/osfans/trime/ui/main/MainViewModel.kt

@@ -0,0 +1,23 @@
+package com.osfans.trime.ui.main
+
+import androidx.lifecycle.MutableLiveData
+import androidx.lifecycle.ViewModel
+
+class MainViewModel : ViewModel() {
+
+    val toolbarTitle = MutableLiveData<String>()
+
+    val topOptionsMenu = MutableLiveData<Boolean>()
+
+    fun setToolbarTitle(title: String) {
+        toolbarTitle.value = title
+    }
+
+    fun enableTopOptionsMenu() {
+        topOptionsMenu.value = true
+    }
+
+    fun disableTopOptionsMenu() {
+        topOptionsMenu.value = false
+    }
+}