Add the possibility to select whether or not to synchronize the files in a folder

Right now it is possible to select which subfolders you want to synchronize from a given folder. But you cannot indicate whether or not you want to synchronize the files in this folder. For example with this tree:
```
Folder
+- Subfolder1
+- Subfolder2
+- Subfolder3
+- File.jpg
+- Bigfile.iso
+- Bigfile.log
```

If I choose `Folder` as the root of the synchronization I can choose whether or not to synchronize `Subfolder1`, `Subfolder2` and `Subfolder3`, but I cannot choose whether or not to synchronize the files contained in `Folder`. It would be very interesting to have an option to indicate whether or not you want these files.