improve event_emitter
- add once
- _signal = fast _emit without side effects (preventdefault)
- capturing argument for addListener
add afterExec event
improve MouseWheel behavior
move textarea and gutter-line-highlight to main cursor
fix typo in fold_handler
add beforeRender and after render events
when text is wrapped highlight full gutter row and screen row
Issue #1232: Scroll editor into browser view on cursor movement