WIP: js: Avoid toggling reactive during animations [performance]

The reactive property was being toggled approximately 50 times (twice for each icon plus the AllView) during the spring animation. And mostly at different times as icons completed the animation at different times, so not in the same frame.

The problem was that toggling reactive on an StWidget incurs a style change of the insensitive pseudo class, and style changes would quickly queue relayouts incurring full stage reallocation. This occurred many times over the course of one spring (or pulse) animation hogging the CPU and limiting the frame rate.

We need the insensitive pseudo class to work, so changing that in response to reactive is correct. However we don't need to do the reactive toggling at the start or end of the animations because those always start or end with the actor changing visibility too.

So by removing the reactive toggling during animations we can avoid the CPU burden of multiple full stage relayouts mid-animation, and achieve a higher frame rate.

Results from one machine:
Before: 44 FPS, relative CPU time painting: 11%
After : 55 FPS, relative CPU time painting: 20%

TODO: Find out what the remaining causes of relayout/allocation are (seem to be direct calls from JS). And if possible add a second commit to reduce those.

Helps with mutter#232 (closed) and mutter#233 (closed).