InputService: add basic ASCII keyboard handling

[bk138]

This implements additional keyboard input from VNC viewers in the form of:

• ASCII characters, i.e. basic typing
• Left and Right arrow handling to select text cursor position
• Delete and Backspace handling for editing text
• Enter/Return handling on API level >= 30

All display-specific, i.e. provides one keyboard focus per display.

Closes #4, finally.