Reading mode truncated for recordings below 60 s

[kdarras]

For recordings that are shorter than the duration into which the reading mode focuses, clicking the reading mode button will display the entire spectrogram but a wrong total duration:

Also, playback will stop after the end of the file has been reached, and the cursor position will not be consitent with the true position inside the file.
The desired behavior is that for these truncated last views, the frame is shifted less so that the end of playback and of the spectrogram coincide. Correspondingly, the cursor would start at a variable position after the start.
related to #295