CR for Job #20140 - Add Overlays.getTextWidth

Code Review for Job #20140 - Workitem available at https://www.worklist.net/20140
The QMap::contains(key) call requires a lookup in the map and the QMap::operator[key] call does another duplicate lookup.  Since QMap::operator[key] will return NULL when the key is not in the map it is possible to get the work done with a single lookup and a check for NULL, and would be faster.

Also, change **0.f** to be **0.0f**, as per coding standard section 3.5.3.