Playlists: enforce update of playlist labels

[ronso0]

For some reason (I failed to find) the sidebar labels aren't updated anymore in main (with Qt6) when adding a track to a playlist.

• First commit fixes a stupid mistake (mine) made in fix/History: remove obsolete placeholder playlists #12494
• second commit adds explicit m_pSidebarModel->triggerRepaint() (like CrateFeature does) to enforce the label update

Closes #12761

[ronso0]

Still it would be good to know the root cause since @m0dB says this was working fine when they worked on #12020 (based on main). I'll try to find a working ref in main (some day in Sep '23) and bisect (assuming this is a Mixxx issue because I'm pretty sure my Qt6 source didn't change).

[ronso0]

I'll try to find a working ref in main (some day in Sep '23) and bisect (assuming this is a Mixxx issue because I'm pretty sure my Qt6 source didn't change).

I checked out a ref where 2.4 was merged to main in August but that also had the issue.
I case someone wants to dig deeper, go ahead. But for now I think this fix is okay, maybe even more sustainable because of the explicit repaint.

[m0dB]

I agree. The solution is clear and clean and I don't think it's worth it to invest more time investigating this. Thanks for the quick action, @ronso0 !