Fixing navigator centering bug with qt 4.7 on mac

This one is a bug correction for Mac release. When compiling for Mac, we have to dodge many of qt bugs and qt 4.7.0 is the best one I've found so far. 

However, in the current release and dev version, there is a bug in qt 4.7.0 that makes the patient navigator view not centered in its column as it should be. 

This PR introduces a correction, specific to Mac and qt 4.7.0, that makes the navigator centered again