during theme switching

- Adds reset method to ExpandableCompositeElement to support
css engine resets for titleBarForeground
- Adds test for ExpandableComposite titleBarForeground reset

Change-Id: Id072ea2f53745dd3e1074fee6e5f13fc320bda18
Signed-off-by: Fabian Pfaff <fabian.pfaff@vogella.com>