FmDndDest: accept drop from libfm-demo to pcmanfm and vice versa.

We never check GTK_TARGET_SAME_APP in fm_dnd_dest_find_target() but we
should check it.
[#3614629]the "text/uri-list" should be primary target for DnD source.

Some applications check only first target and never check all targets list
and therefore don't accept our drop (FM_DND_DEST_TARGET_FM_LIST)
despite the fact we support few other targets as well.