Improve Exit prompt UI for active torrents

[JNavas2]

Please provide the following information

qBittorrent version and Operating System

4.1.6 64-bit on Windows 8.1 Pro 64-bit (6.3.9600 Build 9600)

What is the problem

Exit prompt when there are active torrents gives choices of Yes, Always Yes, No, which is poor UI, because it's too easy to hit Always Yes by mistake, with no confirmation or obvious recovery.

Edit: Added currently presented prompt screenshot.

What is the expected behavior

Better UI is to change Always Yes button to Don't Ask Again checkbox, as in other parts of the UI. See attached example.

Steps to reproduce

Exit qBittorrent with active torrents and default settings.

[xavier2k6]

@thalieht @FranciscoPombal => feature request