Closed file converter, but files still converting.

[HaikenEdge]

Describe the bug While running a video conversion, I Xed out of the window, thinking it'd cancel all the tasks, but it doesn't; it just closes the window and keeps converting.

To Reproduce Steps to reproduce the behavior:

1. Start a conversion.
2. Hit X in the upper right corner to quit.

Expected behavior Exiting out of the window should end all conversions immediately and clear the queue.

Desktop (please complete the following information):

• OS: Windows 10 64bits

[Tichau]

linked to #429