Added -common, -xorg and -wayland subpackages

I have splitted nemo-mobile-session in three subpackages:

- common: has files needed by both configs (room for future improvement here, as I'm not sure which env vars and which files are X11 only)

- xorg: keeps x11 session files, pulls in xorg-launch-helper. This is the package current images will automatically upgrade to.

- wayland: Wayland specific files

It has been tested on an updated X11 image using "zypper dup --from" to bypass the vendor check.

We will have to edit patterns accordingly, pulling in nemo-mobile-session-xorg/wayland instead of nemo-mobile-session