Combine multiple sharees

[tobiasKaminsky]

#3320 is implemented nearly everything from nextcloud/server#12874 (comment), but it is missing the stacked sharees.
This is due to the fact that this info is not yet available via dav.

To finish and merge #3320 I'll create this issue to not forget about it.

[nextcloud-android-bot]

GitMate.io thinks possibly related issues are #3029 (Handling of conversation shares), #3402 (Share file problem), #2705 (How to copy share link), #677 (Sharing multiple files to other apps), and #2046 (Sharing encrypted folder).

[AndyScherzinger]

#3320 has been merged, shipping with 3.6.0

[tobiasKaminsky]

We have this since a while.