WIP: Tabbed windows

This is some really old code that I brought back to life in case anybody wanted to try to bring tabbed windows back. Half of it was from old KDE 4 support and part of it is from a work in progress of Martin.

I will keep this MR updated with what works and what doesn't, some details plus I'll always rebase the code to master if anybody else wanted to give this a shot.

Depends on kdecoration!1