lttng: Some performance improvements to the Resources view

lttng: Some performance improvements to the Resources view

On Linux/GTK, calls to getSystemColor, getFontMetrics and gc.stringExtent are
quite expensive. This patch minimizes those calls. As a future improvement,
we should look into optimizing calls to TimeGraphControl.getXForTime because it
calls Control.getSize() which is slow as well.

Change-Id: I63460e305d8de458dbee4b6cb162b6cf04e77054
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/34743
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>