[WIP] Custom title bar

[karashiiro]

I figured most UI styles we might want to try would require a custom title bar, so I copied this out of the docs and tweaked it a little.

Limitations that I may or may not address now:

• Dragging the actual title text doesn't drag the window (probably not going to use the text anyways)
• The scroll bar is gone
• The window has no drop shadow, because apparently you can't disable the window border and still keep the default drop shadow

I'm probably going to move onto a different part and come back to this later, merge it if you feel it'll be useful.