Only report ID3 when the content changes

[argilo]

Fixes #429.

In general, libnrsc5 reports events only when the corresponding data values change. This avoids burdening applications with duplicate events, and cuts down on the amount of information that the command-line applications log.

But until now, ID3 events did not get this treatment. Every ID3 tag in the PSD stream was reported, even if it was the same as the last one. In some cases, multiple events were reported back-to-back because a single PDU can contain multiple ID3 tags.

Here I've changed libnrsc5 so that it only reports ID3 events when the PSD data changes. This roughly halves the amount of information logged by the command-line applications.

[argilo]

@TheDaChicken noticed two issues:

1. output_reset leaks memory because it fails to free last_psd. I'll fix this by adding the missing free.
2. When switching between programs using keyboard commands in the CLI apps, the current PSD information is not printed. To solve this, I suppose the CLI apps will have to store the most recent PSD information for each program. A getter function in the API (as proposed in Getters for event data?  #462) could make this easier.