Allow to activate Panel through a keyboard shortcut

Navigate panel using keyboard
It is possible to assign a global shortcut to a panel in particular or using one that cycles trough all panels.

This MR provides the following
- Allow panel to accept keyboard focus, (setting the shortcut is done in this MR plasma-workspace!1143)
- Allowing Compact applet to gain focus when Plasmoid.keyboardActivationEnabled is true 
- Add decoration when an applet is focused ![image](/uploads/56c14019ef664d2dc84a410d55eb456c/image.png)
- Allow the task manager to take focus on single tasks, which will activate the window

Related merge requests:
- 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
- Plasmashell/Systemtray/Activitybar plasma-workspace!1143

Co-authored with Benjamin Port<benjamin.port@enioka.com>