Make updating info faster for multi-cursor edits

[as-cii]

With this PR, we take advantage of the fact that the information we present in status-bar is only based on the last cursor and selection. This means we can ignore most of the selection/cursor change events and only update the UI when the last event arrives.

Before

(Total: ~ 330ms, benchmarks performed by editing 3000 lines, inserting 5 characters)

After

(Total: ~ 7ms, 47x faster 🐎)

Some thoughts

I wondered if we had to expose richer events in TextEditor and avoid to perform this last cursor/selection comparison, but ultimately I believe it's fine to let the clients decide when to perform certain events without cluttering TextEditor's public interface with events that can be synthesized pretty easily.

/cc: @nathansobo @maxbrunsfeld