RTL support /1
MyDash: add funtion to set the max size from outside.

Intead of relying on a constrain on the dash actor. I can't set this actor size
as it would interfere with the other containers dealing with input regions.
Set dash max size instead of relying on a constrain.

This solution look simpler and the previous one was giving some problem in
horizontal orientation and rtl language.
Hide The overview dashSpacer whrn the dash is not on the primary monitor.
StaticBox update is linked to the slider allocation.

As the statiBox is computed also from the slider position, connect to the
slider allocation change instead of the dash. Both are as wide as the dash.