Usability: Provide a cancel button for the transcription process

[jdittrich]

When transcribing a file, the screen looks like this.

Problem: There is no way for the users to cancel the process (See Nielsen heuristic 3) in case they decide they want to use another model or that they do not have time etc.
Possible solution: Provide a cancel button, maybe with an additional warning-popup upon click (The process runs since… are you sure you want to cancel?)

[candideu]

Agreed! Related issue: #457