Bug 452138 - Reduce number of error-prone manual steps needed to support
new versions of XULRunner

Provide an overload of VtblCall which can generically call the majority
of methods used by SWT. Provide a way for interfaces to invoke VtblCall
without having to manually provide vtbl offsets.

Change-Id: Ide35e0e258d0903319ca4e3e9789362148480ce6
Signed-off-by: Neil Rashbrook <neil@parkwaycc.co.uk>
Signed-off-by: Lakshmi Shanmugam <lshanmug@in.ibm.com>