Use gapplication tool to launch the application from legacy environments

The launcher binary is not part of the upstream binary package,
so use the "gapplication" tool instead, provided togheter with libgio.
rpm: build rpms in a clean environment

Prevents paths and library sonames from jhbuild to creep in the
final RPM.
I also looked into mock, but it's quite Fedora specific, while
this should allow one to build RPMs that work everywhere - well,
if you ignore glibc ABI issues...
Switch to use systemd for service and DBus activation

The future is systemd running off kdbus, which means that
legacy DBus service files get replaced by a combination of
systemd .service and .busname.