timing: Make scatter viewer update only once

Before, it was updated when the parent's windowRangeUpdated requested an
updateContent and when the viewer itself requested and updateRange, also
in the windowRangeUpdated signal. Now the updateRange method overrides
the updateContent, so the update is done once only.

Change-Id: I8b92367f441c0db62e70e7cc7f2a7c7251b2e598
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/83526
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Hudson CI