Refactor binding accessor

Refactor binding accessor

Change the primary binding accessor to `get_binding_from_xcb_event`.

This function gets a binding from a generic xcb event of type KeyPress,
KeyRelease, ButtonPress, or ButtonRelease by determining the input type
(keyboard or mouse), the modifiers pressed from the filtered event
`state`, managing the proper fall back in case mode switch is enabled,
and finally querying the bindings for a binding that matches the event.

The logic of querying keyboard bindings is not intended to be altered by
this change.

The general accessor has been slightly modified to work with mouse
bindings and made private because it is only used in bindings.c