Introduce Base class for all classes that need access to the Browser

[adiherzog]

Some classes are not PageObjects, but still need access to the Browser: e.g. Prompt, which serves as a wrapper for Selenium Alerts.

It would be nice if classes like that didn't reference the Browser singleton directly through BrowserResource.getBrowser(), but instead through a getBrowser() method defined centrally, i.e. in a common base class.