Improve performances to prevent GUI freezes/slowdowns in the systray

Similar to what has been done in NXDRIVE-2221, we could make the workload lighter for the GUI.

Improvement 1

Review how progression is displayed and computed. In view.py, the TransferModel is not optimized and we should get rid of PROGRESS_METRICS for SIZE, TRANSFERRED and SPEED. Check how it is done for DirectTransferModel.

Improvement 2

When retrieving transfers from the database to display their progressions, we are doing many "conversions": QML > C > Python > C > QML. Each transfer is first retrieved from the database as a dict, then converted to either a Download or Upload object. That object is then converted again to a dict and sent to QML.

We should simply get exact data we need from the database as a dict. And that's it.

Improvement 3

When other improvements will be done, we could enable back real-time count (disabled with NXDRIVE-1839).