This PR introduces timeouts for keyboard modes and multi-key mappings.

A timeout (in seconds) after which an active mode or incomplete key sequence is automatically cancelled can be configured:

• Globally via the map_timeout option (applies to all multi-key and modal mappings)
• Per-mode using the --timeout flag (overrides the global setting)

A timeout value of 0 (which is also the default), disables the timer which means the sequence or the mode will be active until manually cancelled or completed.

For multi-key sequences, the timeout is restarted after each valid key press.

I have tested a dev build with the following combinations of global and per mode timeouts:

1. no timeout configured at all.
2. global map_timeout set, but no --timeout set for any mode.
3. --timeout configured for a mode but no global map_timeout specified.
4. map_timeout set to 0.0 and a non zero --timeout set.
5. --timeout set to 0.0 for a mode but a non-zero map_timeout set.
6. map_timeout and --timeout both configured and non-zero.
   Everything worked as intended.