[Rust - libgui] Implement scroll bar drags

[Rust - libgui] Model a left-click-ended event

[Rust - agx] Cleanly interpolate scroll bar from ‘tuck in’ to expande…

…d scroll

[Rust - libgui] Model state for handling scrollbar drags

[Meta] Rust builds always link with libc

[Rust - agx] ScrollView can compute its content frame

[libgui - Rust] TextInputView passes through Bordered calls to its un…

…derlying TextView

[libgui - Rust] Derived Bordered implementation forwards more trait m…

…ethods to the underlying View

[libgui - Rust] Initial sketch of scroll bars

[libgui - Rust] ScrollView’s interior View does not have a border