Allow to activate Panel through a keyboard shortcut

This MR is part of an effort to allow to navigate panel using keyboard

This MR provides the following

• Allow panel to accept keyboard focus, (setting the shortcut is done in this MR plasma-workspace!1143 (merged))
• Allowing Compact applet to gain focus when Plasmoid.keyboardActivationEnabled is true
• Add decoration when an applet is focused

Currently keyboard navigation was implemented for 4 plasmoids:

• Show desktop, pager and taskbar (in this MR) (depends on https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/439)
• Quicklaunch/QuickShare kdeplasma-addons!107 (merged)
• Plasmashell/Systemtray/Activitybar plasma-workspace!1143 (merged)