Zooming in song editor doesn't zoom to part under cursor

[MightyPork]

This is more of a "missing feature" than a bug - should be easy to fix.

When you zoom in in the song editor, it simply changes horizontal scale without moving the scrollbar, and the thing you were trying to zoom is usually somewhere off the screen. It'd be great if it changed h-scrollbar to keep the under-mouse part visible.

Here's the bug captured (YouTube video)

Expected behavior (YouTube video) (after some video cutting with kdenlive ^^)

[LocoMatt]

I agree with @MightyPork it's very annoying.

[Umcaruje]

Took the liberty of updating the original post with gifs made from the videos.

[PhysSong]

Implemented in master via #3835.