Add an on-screen numpad for use on any smartphone

[sspanak]

There is no good way of using Traditional T9 on a touchscreen-only device. So let's add an on-screen numpad to make it usable.

Settings screen

• Change the "On-screen keys" option to toggle the soft numpad.
• Rename it to "On-screen keypad"
• Make it a dropdown with three options: "Disabled", "Control keys only", "Full-sized".

Design

The layout is similar to Gboard's GODAN.

===================================================
| suggestion | are | taller | like | in | Gboard |
===================================================
|    <    |  Clear  | Filter_ |    >    |
==================================================================================
|               |   1           |   2           |   3           |       ⌫       |
|              	|       ,.?!()  |       abc     |       def     |               |
+---------------+---------------+---------------+---------------+---------------+
|               |   4           |   5           |   6           |      OK       |
|               |       ghi     |       jkl     |       mno     |               |
+---------------+---------------+---------------+---------------+---------------+
|       ⚙      |   7           |   8           |   9           |      🪐       |
|               |       pqrs    |       tuv     |       wxyz    |               |
+---------------+---------------+---------------+---------------+---------------+
|  +Add word    |   *           |   0           |   #           | ⌨️ T9/ABC/123 |
|               |               |   ␣ / +%*{}   |               | ⇧ Shift       |
+---------------+---------------+---------------+---------------+---------------+

• Suggestions row
  • Contains the suggestion list
  • Is as tall as in Gboard
• Suggestion controls row
  • Button meanings: Previous suggestion | Clear stem filter | Set stem filter | Next suggestion
  • "Filter" is in bold+italic, but the underscore is not.
• Left control column
  • Button meanings (bottom to top): Add Word, Show Settings
  • Consider putting only an icon for "Add Word".
  • Consider adding a "left arrow" for moving the cursor to the left. (after Add cursor moving by DPAD #157)
• Right control column
  • Button meanings (bottom to top): Input Mode, Next Language, OK, backspace
  • When the mode changes, the letters on the numpad have their text case (or language) changed accordingly.
  • Consider adding a "right arrow" for moving the cursor to the right. (after Add cursor moving by DPAD #157)
• Numpad
  • If there is enough room put the number on the left and the characters on the right. If not, put a small number in the upper left corner.
  • The character list could be centered or aligned to the right with some padding
  • 2-9:
    • First five characters for this key are displayed
    • Button text case changes according to the current state
  • 0 and 1:
    • Either display the first five or a meaningful hardcoded list
  • # and *: Just type the respective character
  • "Upside-Down Keypad" flips the number keys upside down.

Refactoring

• Create a handlers directory inside ime.
  • ime/KeyPadHandler -> ime/input/KeyPadHandler
  • ime/SoftKeyHandler -> ime/input/SoftControlPadHandler
  • create `ime/input/SoftNumPadHandler

Second phase

These should probably become separate issues.

• Vibrate on keypress
• Suggestions row
  • Touching a suggestion selects it
  • Sliding, scrolls them - the RecyclerView supports it by default
• Right control column
  • (Not sure about this yet) Input Mode shows the current mode: T9, ABC, abc or 123 (or maybe the next mode?).
  • (Not sure about this yet) Consider changing Input Mode icon and text to ⇧ Shift, when composing a word.

[sspanak]

Took a slightly different approach with the layout and everything.

There remaining stuff is:

• Restore the key inscriptions.
• 123 mode does not work at all.
• Verify the candidate colors look fine in the small main menu.
• Verify the small menu is still working fine in general.
• Pressing BACK must hide the keyboard, then when pressed one more time, navigate back. This is for consistency with other keyboards.
• Update the documentation.
• More meaningful icons or text for the control keys (Add word, mode, etc).
• Add Move left/Move right will be done in Add cursor moving by DPAD #157

These will be done in #240:

• In Numeric Mode: hide the "Next Language" key and show "," and ".".
• Holding Backspace or 0-key does not work
• Add suggestion control keys in a new row, right below the suggestion bar: Filter/Clear filter, Select Previous/Next (after Add cursor moving by DPAD #157, when there will be separate functions for filtering)
• Vibration on key press (+ a setting)
• Add proper support for the "upside-down keyboard" setting.
• Add "Key Hints" Setting. When enabled, assigned hardware keys would be displayed above the software key icons (or labels)