[core] Use grl_plugin_registry_get_sources_by_capabilities() instead of checking for virtual method implementations directly.
[core] Use grl_plugin_registry_get_sources_by_capabilities() instead
of checking for virtual method implementations directly.

MathieuDuponchelle/grilo,kyoushuu/grilo,kyoushuu/grilo,kyoushuu/grilo,grilofw/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,jasuarez/grilo,MathieuDuponchelle/grilo,GNOME/grilo,grilofw/grilo,kyoushuu/grilo,grilofw/grilo,jasuarez/grilo,GNOME/grilo