At the moment we need to throw the parsed information away and reparse
everything if we add another mime xml file (*), so make sure that we do
not use mime information (which triggers a first parse) before we added
all mime xml files.

(*) we do that because the data for a mime type is distributed through a
whole range of classes which do not easily allow to remove an already
registered mime type, but we must be able to overwrite mime types from
freedesktop.org.xml.

Change-Id: I5e532875cc750af1da3e0139397279d7be203c27
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>