kcms/tabbox: Manage shortcuts via ShortcutSettings
Leverage the standardized ShortcutSettings
config object to handle
the shortcuts config for the tabbox (setting default values, reading
and saving to KGlobalAccel, etc).
This way, we can remove the extra methods in TabBoxConfigForm
and
improve separation between UI and logic.
It also fixes the actions getting stored untranslated when saving from the KCM (until next restart of KWin)
Depends on: !3543 (merged) and !3544 (merged)