Add auto-hiding panel support to PlasmaShellSurface interface

Summary:
This change adds support for auto-hiding panels. A PlasmaShellSurface
with Role Panel and PanelBehavior AutoHide can request to get
auto-hidden at a screen edge. The compositor will then not show the
surface although it is still mapped and will show it again once the
screen edge gets triggered.

The interface is extended by one new request to allow the client to
request the hiding of the surface. In addition two events are added to
inform the client when the surface got hidden and when it got shown
again.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D3007