Move some gnome@ ports to desktop@

As gnome@ is lacking active committers at the moment, transfer some of its
ports [1] up the stack to the desktop@ group, in hope that this way we get some
updates in as the set of people that "should feel responsible" grows.

As soon as gnome@ grows some committers again, this can (and should) of course
be reverted again.

[1] The list of ports chosen in this move consits of all the ports that are
required to build x11/kde5.

Approved by: portmgr (bapt)
Differential Revision: https://reviews.freebsd.org/D26362