xwm: use last focused window for guessing transient parent

xwm: use last focused window for guessing transient parent

On X the global absolute coordinates are sent in ConfigureNotify and transient
windows are mapped exactly on that position. On Wayland we don't have the
concept of global coordinates, and that's a problem for transient surfaces
without transient_for set because they rely on such hint for setting their
positioning.

So this solution is a workaround. It guesses a parent based on the last
focused window to determine the relative position of the transient surface.
This put transient windows of Chrome browser back to work.

Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>