Add 'Reset On Hide' to Animation part 

Adds a 'Reset On Hide' checkbox to the animation part, that is checked
by default. When unchecked, the model's sequence is not set to 0 when
the animation part is hidden. This lets users switch between sequences
with events fluidly.

Not sure why robotboy's month-old commit is getting pulled along with this, I might be syncing my fork with upstream incorrectly