workspace: Lay out windows based on the real allocation

workspace: Lay out windows based on the real allocation

Instead of doing an entire recalculation of window positions when
sliding the thumbnails box, simply recalculate the position and scale
with basic aspect ratio math. This also ensures that windows won't
miraculously swap positions, even if we reposition windows while the
thumbnails box is expanded.

https://bugzilla.gnome.org/show_bug.cgi?id=694469