[PERFORMANCE] Improve UI performance

Improve UI performance by skipping tasks in the processing queue of HtmlPreviewGenerator
and HighlightWorkerThread. We also introduce a delay in the processing in order to
see if more tasks are coming (e.g. because the user is typing fast).
[PERFORMANCE] Only highlight HTML when visible

Only highlight the HTML source when the view is visible. Otherwise
the highlighting is disabled.
Merge branch 'feature/improve-performance-#40' into develop