QmlProfiler: Make sure animation paint events don't overlap

Previously the animation paint events could overlap if the framerate
estimates were off by more than 1 event. Furthermore the duration could
be negative or 0, with a potential crash in the latter case. Also it's
a bad idea to change the server-provided framerate to suit our estimated
durations.

Change-Id: I14da0855d7248a6ef32509572caa439ebb2d430e
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Don't hide the "Painting" category

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>