This moves all the "default" margins of the welcome pages into a single point (the WizardPanel)
in order to prevent having to re-define it every time.
I've also fixed some incorrect margins that we had
CURA-6057
The whole window approach was just causing to much cross platform issues, so
I converted it into a rectangle and put a mouseArea that eats all the events in the
greyOut background
CURA-6057