Refactor key combination observer and mapping

[csiew]

This ticket was initially meant to replace the Xlib-based keyboard event observer with pynput. It was realised that it would be much clearer to just move the keyboard combinations into a dictionary and map them to simple event names.

• Added key_combs.py keyboard combination event map.
• Replace direct key combination if-else tree with dictionary search.
• Use dictionary to map event names to event functions.