shell: expose shortcut to activateLauncherMenu

And set default shortcuts to Meta and Alt+F1.

This is for migrating all modifier-only shortcuts to kglobalacceld (kwin#205 (closed)). Currently in kwin, Meta is bound to activateLauncherMenu by default (https://invent.kde.org/plasma/kwin/-/blob/master/src/options.cpp?ref_type=heads#L757).

The static_cast is used to handle the overloaded method.

CC @mart @vladz

@teams/vdg