Performance improvements for long lists

All performance profiling was done against FireFox. Chrome is better performing in this case.

9f573f9 - On a long list (e.g. 5000 items) the calls to GQuery.children would add as much as 2 seconds to the time spent in ChosenListbox.onLoad
d12684d - Replacing call to GQuery.find.first with iteration improves performance
c4d8d9d - Result highlighting can be quite slow on long lists. Make it optional