Grouping: Client-side groups

[relative-progressio]

It should be possible, that a user is able to create a group. The group should be saved at the local store. The group has the same structure as the server-side group. The flow should be updated, after the group is saved / edited.

[weidingergerald]

Refinement: Also show "x" for unselecting groups Fix bug for double selection when creating new media group Merge with serverside groups