Refactor Scroll widget to have ScrollBar widgets

[ecton]

On Discord, a discussion about how interactive widgets overrode the scrollbar events made me re-test how Firefox's automatic hiding scrollbars work. Their bars are always interactive above the content.

Additionally, when thinking about the data grid widget (#178), that widget will need to have scrollbars that behave the same as the Scroll widget.

To make this work, the Scroll widget should be refactored into using individual widgets for its scroll bars. These widgets can be drawn after the contents, and this will ensure that they receive mouse events at a higher priority than the scroll contents.

[ecton]

Additionally the Firefox scrollbars look a bit nicer -- they have a very small amount of padding around the thumb and render an opaque background when visible. They also still show the little triangles, although they're nearly unusable given how small they are.