Hiding a category in the timeline view is highly confusing. Previously
the "Painting" category would only be visible if either the model is
empty or at least one painting event was available. Like this it's
always there. In the obscure case of a mixed Qt4/Qt5 application you can
get two "Painting" categories now.

Change-Id: I54db106ed868b7e5d46d0e0ac22b46c0df8be862
Reviewed-by: Kai Koehne <kai.koehne@digia.com>