Gui: Allow to add multiple sync folder connection of the same folder

Gui: Allow to add multiple sync folder connection of the same folder

... or child folders

There is also no real reason to forbid the user from syncing the same
folder to multiple location on its hardrive.
A real use case is when the user uncheck a big directory using "choose
what to sync", but would still like to sync a folder within this disabled
tree. The user can now do this with the "add folder" feature

Since 2.3, we even support syncing the same local folder to multiple
remote folder, so why not allow syncing the same remote folder several
times?

Relates to issue #3645