tmf: Cache the result of gc.getFontMetrics in callstack view

This is a major performance bottleneck on Linux/GTK when a lot of elements are
drawn.

Change-Id: I5259a8910f08b6602801f97fc200b00b7c55c4a1
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/34605
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>