Add scroll boundaries to windows

[scallyw4g]

It would be nice if it were impossible to scroll up/down past the boundaries of the content in windows.

Bonus points for adding a little juice by being able to scroll a little way past the end (or top) and having the scroll lerp back to where the hard-stop would have been, like OSX does.

The code that does the scrolling is in stdlib/src/ui/ui.cpp, UiFrameEnd line 2955