qt: introduce animation scale setting

Every time I set up an Android device, one of the first things I do is to enable the developer mode and decrease the animation scale from 1.0 to 0.5 to make animations faster (I find them too slow). A naive Google search shows that this is actually done by many people.

Similarly, some VLC users may want to opt out from animations, or make them faster/slower. According to my experiments, disabling the animations with a 0.0 factor is not reliable at the moment, but a factor of 0.1 seems to work well.

I limited the range from 0.0 to 10.0, similar to Android (except it is a list there). The name "Animation scale" also comes from there.

We need to fix the places in the interface where 0 duration breaks. In some cases this may be a Qt bug, so we first need to confirm if the breakage is due to Qt.

Request review @chub.