Removing pre-installed kbibtexrc; fixing check state logic for search engines

The configuration file 'kbibtexrc' installed by default in a system folder was
already outdated: most of its keys did no longe serve any purpose. The only
remaining keys were the flags whether a online search engine should be enabled
or disabled by default (even this list was incomplete as not updated when engines
were added). Those default values were refactored into a const static set of
engines to be enabled in fresh installations.

The logic to save the check (enable) state of an online search engine was
slightly broken as it triggered writing an updated check state already during
creation of list items when those were still incomplete, e.g. had no names set.
This commit adds code to block signal generation if list items get updated
during constructions or when icons got downloaded.