Closes #ISSUE

Adds an action and special handling in `KeymapFile::update_keybinding`
for removals. If the binding being removed is the last in a keymap
section, the keymap section will be removed entirely instead of left
empty.

Still to do is the ability to unbind/remove non-user created bindings
such as those in the default keymap by binding them to `NoAction`,
however, this will be done in a follow up PR.

Release Notes:

- N/A *or* Added/Fixed/Improved ...