QmlProfiler: Move timeline prev/next functions to models

That's where they belong. Having them in the view makes no sense.

Change-Id: Ia2a6c8b02804ed8a1e10b0731cd62b6fd6489b0c
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
QmlProfiler: Don't change row count if a model is hidden or empty

This reduces the complexity involved in making the row count a
property. Empty models do have rows like this, but don't have a height.
It doesn't get much more consistent than that. Before empty models
didn't have rows but you could still query the row heights. Having
height == 0 is very helpful for assembling the UI, rows == 0 not so
much.

Change-Id: I38ee9f46751a4beb288578d5cd1f0a17ea08814a
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
QmlProfiler: Make TimelineModel usable from QML

Make the relevant member functions Q_INVOKABLE and properly notify
about the height change when data is loaded or cleared.

Change-Id: I7c1ee70942617af2045b84a2d8f0ab2adef72f52
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>