TMF: Cache average char width to speed up control flow drawing.

TMF: Cache average char width to speed up control flow drawing.

Before this fix, calls to gc.getFontMetrics() represented more
than 40% of the time required to draw the control flow for some
traces
(http://secretaire.dorsal.polymtl.ca/~fdoray/tracing/screenshots/getFontMetrics.png)

With this fix, the method is called only once. Navigation is
much more fluid for traces that contain a lot of state changes.

This fix should also be applied to other similar presentation
providers.

Change-Id: I5dd7815f9ee52e24d6467250fea4e0859be202f6
Signed-off-by: Francois Doray <fdoray.eclipse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/43182
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>