Add cut/copy/paste functionality

[sspanak]

After #157, when it becomes possible to move the cursor with the arrow keys, it would be convenient to select the text using the keypad. Then after #263, the command palette will allow adding new functions easier.

Here is how to do it:

• Add 5-key/"Copy/Cut Text" entry to the main command palette. This is disabled when the On-screen keyboard is active, because there would be no room to display both a keyboard and a command palette.
• Add 9-key/Paste entry to the main command palette. This is disabled when the On-screen keyboard is active.
• Entering text selection mode, activates a new "Select Text" command palette
  • 1-key: select the left text selection border. Once selected it can be moved in both directions with Left/Right or 4-key/6-key
  • 2-key: Select None
  • 3-key: select the right text selection border. Once selected it can be moved in both directions with Left/Right or 4-key/6-key
  • 4-key: Move the current selection border to the left.
  • 5-key: Select all
  • 6-key: Move the current selection border to the right.
  • 7-key: Cut
  • 8-key: Copy
  • 9-key: Paste
  • 0-key: Exit text selection mode

After #157 and #263. Otherwise, this feature makes no sense.