Keybind help widget implementation

[alexm-dev]

Added

• Keybind help: Added new overlay widget with "`?" as default keybind to show current mapped keybinds.

Changed

• Prefix key handling: Changed how prefix key-handling is implemented. Now AppState's central handle_keypress handles all relevant prefix actions.
• Keymap lookup: Possible now to use simple single characters for keybinds instead of ["Shift+/"] or ["Shift+?"].
  Now: ["/"] or ["?"]

Note:

• Keybind help widget still needs work and testing