Smart Playlist: enhance rule "Last playback"

[Laster2800]

Hello.

Now, we can only use the "Last playback" rule with a specific time.

e.g. - [Last playback] - [less or equals] - [2020.01.01 00:00:00]

It's hard to use, there should be a rule like this:
      - [Since last playback] - [greater or equals] - [60](days)

It'll filter music that never played in the past 60 days.
In the current version, we can only change the "Last playback" setting at intervals to do this.

Please implement it, thanks!