Never freeze again.

Put snapshot related floaters always on top of the full screen preview.
Make having multiple running uploads more robust.
Fix infinite recursion when clicking on the bottom 'Snapshot' button
under certain conditions.
Unfreeze time more robust; unfreeze time out of precaution when
starting an upload (if 'Keep open' not checked).
Fixed a bug that caused all callbacks for successful feed uploads to be
ignored.