Loader tool: Separate logic from UI

[iLLiCiTiT]

Description

Loader at this moment has whole logic in each separated widgets but in a lot of cases they should use single controller and model. The best example is subset widget and representation widgets which are both querying the same data and creating modules manger for sync server specific logic. All of that should be implemented using fully UI independent objects (pure python). Each of these steps is slowing down the UI a lot especially when server (mongo) is running on cloud.

This approach would also make cleaner UI.

[cuID:OP-3262]