Fix ditto instance retain cycle in PresenceViewer

[bplattenburg]

Based on @magicwave 's changes in getditto/DittoSwiftTools#167, but in the new presence viewer source of truth.

[magicwave]

@bplattenburg - With this standalone repo would we expect to import the DittoSwiftTools package without the PresenceViewer module and independently import this package? Currently this produces an SPM conflict on import. Or maybe the PresenceViewer module will be removed from DST package?

[bplattenburg]

@bplattenburg - With this standalone repo would we expect to import the DittoSwiftTools package without the PresenceViewer module and independently import this package? Currently this produces an SPM conflict on import. Or maybe the PresenceViewer module will be removed from DST package?

@magicwave The intent is that DittoSwiftTools depends on this package, and customers would only need to import that. getditto/DittoSwiftTools#166 makes those changes to depend on this repository and remove the duplicate copy.