Renamed the source to libfm-gtk3, and subsequently every binary package, using the libfm-gtk3 prefix.

Note: due to this change, and to ensure uniformity, every library package has a dash before the SONAME.
This is intended.

Also, the Build-Depends have been changed so that now they install gtk3 instead of gtk2, and the autogen.sh
arugments now contain --with-gtk=3.