tmf: Fix concurrency issues in histogram view

tmf: Fix concurrency issues in histogram view

- Make sure signals sent to itself with the signal throttler are
executed in the UI thread, so that Histogram.clear() doesn't
concurrently set fScaledData to null

- Clear the drag state when Histogram.clear() is called so that
subsequent mouse operations don't need to protect from clear during drag

- Prevent the trace updated signal from constantly resetting the
histogram range so that trace indexing doesn't interfere with drag move
and drag zoom

- Fix time range in full trace histogram being offset incorrectly when
new trace events cause the scale to change during dragging

- Fix mouse wheel zoom out snapped to current zoom start time in time
range histogram when trace updated

Change-Id: I1c6e71d9c7f26dec93192768e99306fdbb978dd1
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/19341