Add helper for accumulating scroll into discrete delta

This converts `ScrollDelta::Pixels` and `ScrollDelta::Lines` into
integer values, accumulating partial scrolls until a full integer is
reached.

It also has a configurable rate-limit, so discrete integer events can
occur at a certain maximum frequency. This may need tuning for different
use cases, though I haven't tried using it for things other than
changing workspaces so far.

src/lib.rs

@@ -108,6 +108,8 @@ pub mod task;
 
 pub mod theme;
 
+pub mod scroll;
+
 #[doc(inline)]
 pub use theme::{Theme, style};