Add request to have focus in a PlasmaShellSurface of Role Panel

Summary:
By default a panel does not take focus. But there are panels which
should get keyboard focus. Examples in a Plasma session are the widget
explorer.

This change adds a new request to PlasmaShell interface to specify
whether a panel should get focus. The compositor can use this request to
decide whether to pass focus to a panel.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3035