🔒 File locking

[AndyScherzinger]

Implement file locking support on:

• Android (Implement file locking android#10012)
• Desktop (Implement file locking desktop#4382)
• iOS (Implement File Locking ios#1888)
• special case (2 types of lock?)
  • open file in web (e.g.) collabora online
  • automatically lock file for all clients
  • clients then only offer editing online

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• Desktop
  • lock/unlock the document from the right-click menu of the desktop file manager
  • indicate to the user that the file is locked, some ideas:
    • overlay icon on the file
    • remove the write permission on the filesystem on lock and add it again on unlock
    • Notifications which pops up if a file was locked, might be useful if the user has the document already open but forgot to lock it, so that they know than now someone else claimed the document
• Android/iOS:
  • lock/unlock the document from context menu
    • when locked, show icon, read-only actions are possible
• special case: open file in e.g. collabora online: lock file on clients, only allow editing online

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Server issue: nextcloud/server#21914