Fix #343 videopassthrough plugin framerate not set on load

Since we now defer setting up Qt connections until after the
plugin widget is fully loaded we need to set both the slider
and spinbox values on load so they are in sync. Also fixes mixer
inputs not showing configuration icon on initial loading of
the widget.

Fix #344 Mixers input boxes do not show configuration icon

Signed-off-by: Thanh Ha <thanh.ha@alumni.carleton.ca>