[Desktop] Placeholder improvements

[TheOneRing]

There are several issues with the current implementation.
This lies in the nature of the current implementation.
Placeholder files hold no data, only the file name provides data.
If we would write the original file path and hash to the file, we could detect a move when encountering placeholder in a new location....

• Data loss when switching on VFS for .owncloud files client#7557
• Wrong notification when a placeholder is renamed  client#7327
• [VFS] Branding name should not influence the placeholder file .owncloud client#7677 ...

[michaelstingl]

Placeholder improvements not planned. Efforts should go into native VFS implementations.