Consistent management of project CRS properties in project file

Following bug 9397 http://hub.qgis.org/issues/9397, I made project CRS properties storing more consistant. 

Previously, only "ProjectCrs" was created at the project creation. "ProjectCRSID" and "ProjectCRSProj4String" were only created when a different CRS is chosen, but saved in two different places. Also, "ProjectCrs" was not updated once a different CRS was chosen.

With this patch, all three properties are created as soon as a project is saved, then all three are updated in the same function when a new CRS is chosen.