kcms/tabbox: Manage shortcuts via ShortcutSettings

Leverage the standardized ShortcutSettings config object to handle
the shortcuts config for the tabox (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)