QmlProfiler: improve selection behavior in timeline

When selecting ranges in the timeline the selector would sometimes hang
or behave weirdly when moving back. This was due to incorrect logic in
the selection bounds calculation and because the vertical flicking would
steal mouse events.

Change-Id: I14074463422d1d9a0aa8ecd1f88847e7330c9b6b
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
WelcomePage: Remove outdated tutorial videos

Remove following videos that use outdated terminology:
    - Qt Quick Elements, Part 1
    - Qt Quick Elements, Part 2
    - Qt Quick Elements, Part 3

Task-number: QTBUG-35187
Change-Id: Iacecf06b4868ecbc71fa2b7d3cc3ad23533e88b4
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>