Add wayland decoration support

[mechtifs]

Since GNOME fully turned to CSD on wayland, decorations on Qt apps are not rendered as they are on x11. Currently by setting QT_WAYLAND_DECORATION would specify CSD for Qt apps, as seen on adwaita-qt.

Would it be possible to support this feature?

One thing to consider here would be the possibility of "true CSD" via a QT theme engine. Three ways it could be done (not necessarily mutually exclusive):

• inject window controls into the toolbar/menubar directly. Kvantum already allows you to drag windows from anywhere in the toolbar. What's missing are the window buttons and the ability to double click/right click on empty space to maximize/restore.
• export the menubar (as menubar, hamburger, and/or individual menu items) into the decoration. This seems like it might be possible via unity global menu functionality in QT. How doable it is I don't know but theoretically it could be used to graft headerbars onto the app, maybe even custom headerbars if you can customize how the menu hierarchy appears.

Obviously this can't be enabled by default because it might break apps. But if it works it could be used to transform Qt applications into Gnome apps.