qt: do not take snapshots from the UI thread

Snapshots are performed via a call to:

    var_TriggerCallback("video-snapshot")

causing its callback (SnapshotCallback) to be called synchronously,
executing the following steps:
 - wait for the vout thread to actually capture the (next) frame;
 - encode the picture to PNG;
 - write the result to disk (I/O).

Since var_TriggerCallback("video-snapshot") is called from the UI
thread, all these blocking actions are also performed on the UI thread.

Move the call to a separate thread.