Migrate away from OwnCloudClient to NextcloudClient [1]

[ZetaTom]

This issue serves to outline and track the changes made to migrate from the legacy OwnCloudClient to the current NextcloudClient.

As this endeavour requires a lot of mostly minor changes it has been decided that it shall be split into several "smaller" pull requests.

Part of this first set of changes are the following:

• Deprecate OwncloudClient - Helpers #1271
  • helper classes to ease the transition and refactoring
  • general changes
• Deprecate OwncloudClient - Notifications #1272
  • Deprecate OwncloudClient - Notifications android#12266
  • changes related to notifications
• Deprecate OwncloudClient - DirectEditing #1273
  • Deprecate OwncloudClient - DirectEditing android#12267
  • changes related to direct editing
• Deprecate OwncloudClient - Sharing #1294
  • Depocc/sharing android#13178
  • changes related to sharing
  • server status and info operations
• Deprecate OwncloudClient - Comments #1295
  • Deprecate OwncloudClient - Comments android#12464
  • changes related to comments