Bug 987237 - Add option to display appIcon in cards; make screenshot preview in cards a settings option. r=alive

* New default/fallback icon for card view

* Use span for fallback cards view icon

* Move early return to ensure non-screenshot cards still get rotation class

* Implement cards view icon as overlay and fallback when screenshot setting is false, or retrieval fails

* Fix up getIconURI per bug 988128

* Add screenshot cards view setting to developer menu, with initial/default setting value as true

* Add tests for enabled and disabled screenshot handling in cards view

* Add some getIconURI tests

* Use SettingsListener to awatch screenshots.enabled setting; update test