Changes done for GEB-310 - Support navigating to page instances in addition to classes

Following changes were done:

- Extracted Interfaces for all the support classes.
- Added Uninitialized support implementations for the said interfaces
- Added PageInstanceNotInitializedException class.
We want a test like that for one method from every support types used by `Page`. You can use `@Unroll` and call methods like this: `parametrizedPage."$methodName"(*args)` where `methodName` and `args` are specified in the `where` block.