Compare track length when merging tracks

[robbevp]

Is your feature request related to a problem? Please describe.
As a user I don't want to accidentally merge the wrong tracks. We often merge a whole album at once, so merging becomes a repetitive task than might lead to small errors.
When merging two tracks, we probably have a new rip of an album or replace an MP3 with FLAC, .... The track length will often (but not always!) stay stable.

Describe the solution you'd like
We should also compare the track length when merging tracks
either by:

• adding an extra row to the comparison table
• showing an alert when the length of the tracks does not match.