Wayland integration: support for server-side decorations

Summary:
If the application runs on Wayland, check whether the
ServerSideDecoration protocol is supported. If that's the case, disable
Qt's decoration and tell the Wayland server to use Server side decoration
for normal windows and no decoration for popus, etc.

Reviewers: mart, sebas

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D742