Add support to keyboard shortcuts inhibitor

Summary:

• Disable all global shortcuts
• For now don't disable spies (So opening menu with meta key is not disabled for now)

Depends on D29231

Test Plan: Tested with https://github.com/swaywm/wlroots/blob/master/examples/keyboard-shortcuts-inhibit.c

Reviewers: davidedmundson, zzag

Reviewed By: davidedmundson

Subscribers: romangg, broulik, apol, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29272