Add more customization to the new window close animation

[Diax170]

for example make it togglable, add its own animation curve, or make it also shrink the window while/instead of fading

[dawsers]

I only added this because I had to delay transaction destruction for other reasons. That made the view data available until the transaction is finally destroyed (for example, at the end of the current animation). So the possibilities to customize this are limited.

Changing the size of the window while the animation is running is not possible. The containers are no longer valid, and they don't belong to the scene graph or the workspace list, that is why I opted by a fade animation. I think it is also the default for other compositors.

In ebbfb12 I added the ability to customize the fade_in (opening) and fade_out (closing) animation curves. These are complementary to window_open and default (when closing), and only affect the fading of the opening and closing windows. You can disable them and won't get any "fading".