qv4l2: correctly resubscribe events after a reopen.

qv4l2: correctly resubscribe events after a reopen.

The reopen() call closes the original filehandle and at the same time any
subscribed events. So resubscribe the events.

In addition both the ApplicationWindow and the GeneralTab had a cv4l_fd
instance with the same fd. With reopen that means that those fd's are
no longer in sync.

GeneralTab now has a pointer to the cv4l_fd of the ApplicationWindow, so
they remain synchronized.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>