UI - UserControls style support

[LambdaSix]

Ability to define some kind of UserControl object that would basically act like a Panel with child elements would be very useful for defining common parts of the UI like extended information on Construction/Crafting/Examine windows, where they should ideally be the same in all cases.

However the current design of controls in OctoGhast.UI doesn't allow arbitrary child elements, all elements are added at the top level to the owning Window (which derives from Widget and implements user input) -- so this feature requires ControlBase to be modified to pass user input downwards to its children.

Potentially any UserControl objects would have to pass user input along to their children controls as well.