Allow Panel and systray keyboard navigation

Allow the Panel to get active keyboard focus via a kwyboard shortcut which will cycle between all panels.
When a panel has focus, the active focus can be navigated around either with tab/backtab or arrow keys.
Simple popup applets which only display an icon will work automatically, 
Complex applets like the System tray or the taskbar will have focus that can navigate on all their sub-elements, to activate a particular systray applet or activate a particular window

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

CCBUG: 352476