[FEATURE] Allow provider extensions to be registered using PHP

This feature requires a PageController to be present in the provider extension. If so, the provider's collection can be enabled from ext_tables.php by inserting:

```
Tx_Fluidpages_Core::registerProviderExtensionKey($extensionKeyLowercaseUnderscored);
```

Fluid Pages will then look for template paths in `plugin.tx_thatext.view` - which is already fully compatible with standard Controller paths. In other words, to make any Extbase extension a provider of page templates, simply add a PageController which subclasses Fluid Pages' AbstractPageController and add the registration in ext_tables.php - and you're done, ready to use your PageController exactly like you would use any other Extbase controller with one added convention: one page template selection equals one controller action. Templates are still detected based on files in the path but now, each template in addition requires a corresponding Controller action.