Support forceActiveWindow for Panels

Summary:
On X11 a panel can be activated by setting it to type dock and then
force activating it. This change implements this behavior for Wayland by
checking whether a window refers to a PlasmaShellSurface of type Panel
and then sets the hint that it wants to get focus.

For other window types the force activate is still not supported and
this is also (mostly) the correct behavior.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3036