Easier designing of UI windows

[LambdaSix]

It feels like it would be much easier to do GUI design/layout if the configuration of a screen was moved into some kind of structured file ala WPF's XAML files.

This shouldn't be too difficult to implement even with the current UI code; it would just require modifying the Window -> Control relationship so that Controls can contain child controls rather than only windows containing controls.

Once that's done, writing Container controls like StackPanel or Grid would become a matter of appropriately considered layout rules of their child elements. Data binding of controls to their property values/action handlers can be achieved with reflection or building expression trees for compilation at runtime when the window is first loaded.

At the same time it would be good to move the UI systems over to non-ascii drawing, a current limitation is line lengths and font sizing, layout requires careful consideration of how many tiles wide a window/element is on the screen. Being able to draw fonts directly at any size would reduce this to a manageable level. But that could maybe be another issue/PR.