ManualPlugin: re-enable unhinge button for Qt 5.
ManualPlugin: re-enable unhinge button for Qt 5.

Unhinging works OK now, after we've migrated the plugin
to live inside Mumble.

Fixes mumble-voip/mumble#2456

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble