Loader: Implement existing product group picker in Group Products dialog

[BigRoy]

Changelog Description

Implement an "existing product group" picker in Group Products dialog,

Additional info

Fixes #648

Testing notes:

1. Press CTRL+G in loader to get the Group Products dialog
2. If the current folder has any existing product groups - clicking button right hand side in pop-up should list them for you to easily "put it into that group"
3. if the current folder has no existing product groups - the button right hand side should be disabled.