Emit less signals when reading the qtversions.xml file provided
by the SDK. Especially do not claim a Qt version was removed and
added again on updates. This could potentially cause BuildConfigs
to update themselves in stupid ways.

Change-Id: I5e2b3ac4725f4a30910520e57ca0bbf9fe595f7f
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>