Add option to make exceptions to the exclusion list

Currently, the desktop client can be configured to exclude specific folders from sync based on some pattern in the `sync-exclude.lst` file.

To implement feature as suggested in #4889, it would be great if one could make exceptions.

For example, to allow the syncing of the `.git/config` file while excluding the rest of the repository containing `.git` folder, one could use the following exclusion rules:
```
**/.git/**
!.git/config
```

This should exclude all files and folders within the git repository (and the git folder) from being synced, but should keep the `.git/config` file in the sync. The second line of the rule, starting with exclamation mark, would be a negation that excludes the `.git/config` file from the previous exclusion rule, similarly as in `.gitignore`.

This option would be much more general than making a git specific feature.