GDK W32: Add drag-to-snap feature to AeroSnap

GDK W32: Add drag-to-snap feature to AeroSnap

This implements the part of AeroSnap that snaps windows when you
drag them (while moving or resizing) to the edge of the screen.

It also fixes drag behaviour for snapped and maximized windows
(if such windows are dragged, first they must be unmaximized/unsnapped).

Note that this code does not take into account the shadow width, and
because of that the under-pointer-position-preserving window moves
might not look as such for maximized windows, which lack the shadow
when maximized, but do have the shadow when unmaximized.

This commit also doesn't cover some corner-cases the same way AeroSnap does.

Also, the snapping indicator (which is supposed to be a window shape that
shows where the window will be if the drag op is stopped at its current
point) is not being drawn, all routines responsible for its creation,
moving and drawing are stubs.

https://bugzilla.gnome.org/show_bug.cgi?id=763013