Help: Cleanly separate different viewer backends

Removes the conditional compilation of the text browser vs web kit,
cleans up the code, and enables us to provide and test other backends
(WebEngine? Native?)
Adds runtime switch for testing the text browser variant by setting
environment variable QTC_FORCE_TEXTBROWSER (for now)

Change-Id: I8cc9bb373438d37c8194e433224314caa0b617bd
Reviewed-by: Eike Ziller <eike.ziller@digia.com>