Only show unadded apps when adding apps from remote repos

At the moment when an user adds an app from a remote repo, we show all apps available in remote repos. Instead we should only show apps that are not added to the user's repo yet, so there are only new/unknown apps.

This gets important when we add the possibility to remove apps from this screen in #92 (closed).

@crwinfrey @grote What do you think about this?

changed milestone to %1.0

added bazaar task labels

Good point! Either that or somehow disable the add button so that it is clear that the app has been added already. But maybe hiding is better, I am not sure...

added dev label

I think, I'd go with the disabling the button and showing Added instead of Add, so users can still see that these apps are available in this repository.

assigned to @grote

added Doing and removed ~2112488 labels

mentioned in merge request !97 (merged)

removed Doing label