[Desktop Toolbox] Emit contextualActionsAboutToShow before opening

The toolbox shows containment-specific actions, just like the desktop contextmenu does.
The containment may update or creates its actions on-demand leading to outdated actions showing up in the toolbox.

BUG: 384862
FIXED-IN: 5.12.7

Differential Revision: https://phabricator.kde.org/D14535