Qt4: fix race in requestVideo and simplify

Qt4: fix race in requestVideo and simplify

We were accessing some main interface variables from random threads
without serialization. This simply moves the blocking connection signal
from sole size setting to the whole video widget request call. In the
process, we can remove two signals. We still need a blocking connection
signal. It does not add any new deadlock case, since there was already
blocking connection signal in the same call and in the same direction.