Introduce CGUIColor type inheriting Color type and switch the GUI to exclusively use that.

The type differs from the Color type, because contrary to Color, it can be created from a color predefined in the GUI page (such as "yellow").
Move this predefined color check to the new class instead of hardcoding it in FromJSVal / ToJSVal, GUIUtil, JSInterface_IGUIObject and to straighten the latter.

Delete fov_wedge_color from Minimap, unused since introduction in rP1230.