Fix issue #7645 (auto complete suggestions in dropdown can not be selected in IDE)

As described in https://bugzilla.mozilla.org/show_bug.cgi?id=1005083 the
nsIAutocompleteResult implementation is missing a method to work as intended.

Beside the mentioned ticket the following non duplicate marked tickets can be
closed after applying #7720, #7741, #7737, #7736, #7733, #7724, #7645, #7732
and #7728.