tabwin: Place tabwin in center when display is scaled

Bug: 14931

On scaled display the tabwin is using unscaled window coordinates to calculate
its position. Let the callert choose scaled or unscaled display size and use
scaled in tabwin.

fixes: 9d5ac6a4 Introduce xfwm-common static library

Signed-off-by: Jonas Zetterberg <jozz@jozz.se>