Add '.' (root folder) to selective sync configurations

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Feature description

Currently the client syncs all files under the root directory of a space, even if no sub-folder is selected in selective sync configuration tree view. It'd be better to make the root directory selective too.

Proposed solution

Add '.' or '' or similar thing to the selective sync configuration tree view. If not selected, all files under the root directory are de-synced.

Alternative solutions you considered

No response

Additional context

There could also be an option to make new folders under a specific space de-synced by default.

No file, test, or entry point is named in the issue. Start by locating the selective sync configuration tree view in the C++ desktop client; done means the root entry can be left unselected so files directly under the space root are de-synced.