QmlProfiler: Restrict selection ranges to a minimum of 500ns

Smaller ranges shouldn't be useful for anything but create rounding
errors all over the place.

Task-number: QTCREATORBUG-10880
Change-Id: I5cf5e2c08b5cae676717ba8ada1cddee4523c399
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Properly align time labels and bars

Due to QTBUG-34962 we can't rely on the width properties of Flickables
so we use some other elements to size the background bars.

Task-number: QTCREATORBUG-10882
Change-Id: Iee427022111b6092486adaebfd33f8e2e6a91190
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
QmlProfiler: Remover some dead code

As the timeline cannot show out of bounds areas anymore we can remove
the code which draws the background for them.

Change-Id: Ifedfc10b67bd4ce2e2a4bb2cbdcc03b877472a92
Reviewed-by: Kai Koehne <kai.koehne@digia.com>