[Fix] Fixed issue where keeping 'j' pressed for sometime would *sometimes* cause regular mouse/trackpad scrolling to get "stuck"
[Feature/Perf] Put a mininum time interval between successive invocations of selectPrev and/or selectNext
- This allow better performance/response when user keeps 'j' continuously pressed
- Also, turning of mutation observer around calls to _selectPrev and _selectNext
[Refactor] Renamed variables + minor change