Debian packaging: provide x-window-manager
Remove debian/watch

Leftover of gnome-shell debian package. Unapplicable to cinnamon with Mint.
Remove generated files from versioning
Specify the debian source package format (3.0 native)
Use debhelper sequencer (dh) instead of CDBS

This accelerates the debian package build and is a cleaner way to make
Debian package.