Improve UX for local-only folders

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## Feature request

**Is your feature request related to a problem? Please describe.**
Sometimes there's need to not sync a folder inside a synced folder. In case this folder should stay only in cloud this works well, but if we're talking about local-only folder, it's harder and not obvious to do that. Let's say, I created folder in Video/ with some large videofiles I wouldn't like to be synced. It starts syncing by default. Then if I go to sync settings and uncheck this folder, client says that it will keep this folder in cloud and delete it locally. Then I have to re-create this folder and delete it in cloud - then client remembers that this was excluded from sync and won't upload it again. And then there's no UI to enable syncing this folder back.

**Describe the solution you'd like**
When unchecking folder in sync list client could ask if I want to keep this folder cloud-only or local-only - and then delete it on drive or in cloud depending on answer. It already asks if I really want to disable sync for folder and delete local files, so why not to just improve it?

In addition this could be added as context menu action for file explorers extensions. Also currently files that aren't synced are marked with "Attention" icon in file explorer, and that's right because currently this behaviour is considered as problematic. But if this feature will be implemented, there should be separate icon for local-only files - like crossed out grey cloud.

**Describe alternatives you've considered**
Well, there's a separate action to set sync exceptions where you can also type a name folders/files with will be excluded from sync. But this way requires more actions (open exceptions list, write down your folder name) than just unchecking this folder.

Start with the sync settings folder list and its existing confirmation for disabling sync, then review the file explorer extensions and current “Attention” icon behavior mentioned in the request. Done means the flow distinguishes cloud-only from local-only folders, offers a way to re-enable syncing, and defines the requested explorer action and visual state.

Written by the indexing model from the issue text.