Improves datum transform selection. 

- Fixes issue with "Hide deprecated" check box
- Creates a score to sort multiple datum transform definitions
- Creates a entry on layers context menu to recall datum transform selection dialog
- Prevents multiple mouse clicks on "CRS status" icon on status bar
- Prevents settings update when project properties dialog is cancelled

This can go out. Also the #include <QMessageBox>.
If datumTransformations returns an empty list I don´t known if it is necessary to call 
mSettings.datumTransformStore().addEntry( ml->id(), srcAuthId, destAuthId, -1, -1 );
mMapRenderer->addLayerCoordinateTransform( ml->id(), srcAuthId, destAuthId, -1, -1 );
to setup the default transform. And if it returns a single transform I don´t know if it is the default one.