gnome3: add services.xserver.desktopManager.gnome3.sessionPath

Used to add GI_TYPELIB_PATH and LD_LIBRARY_PATH for gnome shell extensions
or other libraries and services.

Not a great solution but may be the start for further work. Let's make
some cool packages work for now.
gpaste: new package

Clipboard management system with GNOME3 integration

https://github.com/Keruspe/GPaste