Bug 1019905

What about defining a function to explain that a result is an app? This would make this list comprehension lighter, woundn't it?
``` Python
def _is_result_a_webapp(self, result_element):
    # An app result is to an installable (via marketplace) webapp
    return '.webapp' in result_element.get_attribute('data-identifier')

@property
def app_results(self):
    return [self.Result(marionette=self.marionette, element=result)
        for result in self.marionette.find_elements(*self._search_results_locator)
            if self._is_result_a_webapp(result)]
```