Made use of viewMode in core and ui

Backend:
Use `state['viewmode']` when calling API's method `getHeadlines`.

Model:
Update `continuation` depending on `settings.viewMode` instead of
`settings.showAll`.

Sailfish:
Added full support (as full support as backend for now allows) for all
allowed values of parameter `view_mode` in API's method
`getHeadlines`.

Harmattan:
Added support to update backend's `state['viewmode']`, but for now
restricted to `all_articles` and `unread`.
[core] Reorderd settings

ShowAll now only applies to categories & feeds.  Moved it accordingly.