Style Skinning Support

[Stuart Walpole]

[This is pretty low priority, since we can live with using a generic GUI appearance for all civilisations, at least for Part I.]

• Problem: Currently styles are set when the game is loaded, and then cannot be modified. Since the style system is in place, it'd be good if we could use this to allow changing of controls to a different appearance (in particular, a different texture set to "skin" the GUI depending on which civilisation is currently in play).
• Solution: When the style property of a control is changed at runtime, the control's properties are updated to the value of the new style. Also, when the properties of a style are changed at runtime, controls using that style which still have that property are updated to the new property.