kcms/tabbox: Manage shortcuts via ShortcutSettings

Ismael Asensio requested to merge work/iasensio/tabbox-use-shortcutsettings into master

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)

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