VBoxLayout: Fix item height math in ::set_geometry
Size2: Introduce several arithmetic operator overloads

(Mimics functionality found in nom::Rect, nom::Point2 and so on).

* Rect: Add const keyword to operator overloads where appropriate.
UILayoutItem: Rename 'geometry' getterS & setters to 'bounds'

* ::geometry -> ::bounds
* ::set_geometry -> ::set_bounds
Size2: Introduce swap method.
UIWidget: Introduce FocusPolicy enumeration

(These are stub methods for the time being).
examples/gui: Fix positioning coords of listbox layout