Add filter "unassigned cards"

[stefan-niedermann]

For feature parity with server: issue and PR.

• Add a property boolean noAssignedUser to FilterInformation model
  • Update copy constructor and toString
• Respect this property in SyncManager#getFullCardsForStack
  If noAssignedUser is true, the method should return cards which have no assigned user or any of the assigned users in the users list. If noAssignedUser is false, the logic is the same as today.
• UI: In filter dialog, tab "users", there should always be a dummy user at the very top with the label "unassigned" and an appropriate icon as avatar.

[desperateCoder]

see draft PR #595

should work, but couldn't test (no UI)