Fixes a bug where the output resolution wouldn't change when using a
service that uses the "supported resolutions" limitation. When switching
to a non-editable combo box, the signal used to detect whether the combo
box has had its value changed wouldn't work anymore because it was the
signal explicitly used for editable combos. So, to fix, just reapply the
normal combo box signal to detect when the value has been changed.

Fixes obsproject/obs-studio#4124