windowview: handle windows from other virtual desktops better

Sometimes windows on other desktops are higher in the stacking order than windows on the current desktops, which causes them to jump above windows from the current desktop in the opening animation.

To fix that, change the z order to be biased towards windows on the current desktop, and reduce the opacity for windows from other desktops in the beginning of the animation

Note that I had to check for the current desktop manually because thumb.client.onCurrentDesktop doesn't work for some reason. Does anyone know why it's broken?