Support for unlimited audio recording by pressing headset button twice

Added an option to allow for "unlimited" audio recording (well, actually it is limited to a maximum of 10 minutes) by pressing the headset button twice: first to start recording, and second to stop recording. I have tested this option with a real headset, but it should also work with the touch-screen audio note button: when pressed, this button shows up a window that prints the number of seconds of recording, and a "cancel" button. When pressing this cancel button, recording stops - although I'm not sure whether the audio file is deleted or not, I'll check this also.
Anyway, the default length of 2 seconds is preserved, so that users are not surprised by a change in behaviour. I have also seen when looking at the code that there is still much to do about the "audio record" feature: listening to an audio note, deleting an audio file, exporting... I might have a look at all this if you agree, because that's the kind of feature I want to implement anyway for a (future) related project.