Only emit one display layer change event per buffer transaction

[maxbrunsfeld]

Similar to #270, #273

Previously, DisplayLayers would call their onDidChangeSync callbacks once each time their underlying TextBuffer changed. This was a source of slowness when typing with multiple cursors and prevented us from wanting to implement features like auto-indent in terms of a series of small buffer changes.

In this PR, I've renamed .onDidChangeSync to .onDidChange and changed it so that callbacks are only called one time per text buffer transaction. I have also changed the events that it emits so that instead of having start, oldExtent and newExtent properties, they have oldRange and newRange properties similar to TextBuffer.onDidChange events.

/cc @nathansobo