Nicer window shadows

The current window shadows look a bit flat and not very realistic. A relatively easy trick to make shadows look nicer that we employ in other places is using 2 drop shadows instead of one: A darker one with less blur, and a more blurred, more transparent "ambient" shadow. Here's an example of what this could look like:

Note: the slightly darker window border on the right side is accidental, and not part of the proposal

Related: mutter#315