Handle the _NET_REQUEST_FRAME_EXTENTS ClientMessage (java compat)

Handle the _NET_REQUEST_FRAME_EXTENTS ClientMessage (java compat)

This ClientMessage can be used to estimate how big the window will be
before opening it. Java always sends the ClientMessage and checks the
atom that should be set by the window manager, but it seems that the
fallback code path has a race condition.

Let’s see if the situation gets better with this change. I have been
running this patch for about two weeks and have not seen any issues with
it.

fixes #934
fixes #709