Fix class loader mocking in RWTServletContextListener test

To verify that contextInitialized() uses the context class loader, the
test mocks a class loader. When the ThemeManager reads resources during
activation, this class loader is requested to load some xml parser
classes. This leads to an error if the mocked class loader does not
delegate to the RWT class loader.

This error occurs only when running the test stand-alone, otherwise
the ThemeManager is already initialized by other tests.

Change-Id: I2e8379046d0a83faa9ed0026c04cd1f1d206f27d
Signed-off-by: Ralf Sternberg <rsternberg@eclipsesource.com>
Allow overriding ThemeManager in ApplicationContext

For performance reasons, we need to skip the loading of resources for
themeable widget in the test suite.

Allow to subclass ApplicationContextImpl and to override the creation
of the ThemeManager. This results in a cleaner design than patching the
ApplicationContext at runtime.

Change-Id: I7383fbccf3550ffe16607c32f21f4c45d4617d95
Signed-off-by: Ralf Sternberg <rsternberg@eclipsesource.com>