Support scrollable UI nodes (Overflow::Scroll?)

[nicoburns]

What problem does this solve or what need does it fill?

The ability to create a scrollable region is a common requirement when building a user interface.

What solution would you like?

Build in support for scrolling enabled by Overflow::Scroll or some similar marker on a node. It should be possible to control whether a node scrolls independently in each axis.

Additional context

Scrolling is currently implemented as an example in the ui/ui.rs example. But this:

• Requires every user to implement it themselves using raw scrollwheel events
• Does not support affordances such as scrollbars
• Is not a great solution as it requires a relayout every time the scroll position changes