Support docks which take input

Summary:
On X11 one needs to force activate a panel to pass it focus. This change
implements something similar for Wayland but a little bit more stateful
by using a request on the PlasmaShellSurface. If set KWin will activate
the panel.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3037