Allow unsetting a keyboard shortcut.

Also make it possible to override an existing shortcut.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Add basic tests for KeyboardShortcuts.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Clarify confusing comment.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>