Unhide autohidden panel when using global menu

Summary:
This sets a transient parent on the shown menus, meaning a panel knows
to stay open whilst it has any menu showing.

We also pulse setStatus() to trigger activating an autohidden panel if
activated via a shortcut.

BUG: 384861

Test Plan:
Ran menu
Using both mouse and keyboard shortcuts navigated a menu
and submenus

Reviewers: Plasma, broulik

Reviewed By: Plasma, broulik

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D7883