feat: add marquee effect to volume popup's audio inputs

[postsolar]

Part of #1011. Based on #1066 - GTK4 actually fixes that scrollbars bug, so pretty much no changes aside from updating to GTK4 and expanding what's configurable.

Overview

• Wraps the label in a ScrolledWindow with hidden scrollbars
• Configurable max length before marquee starts
• Configurable scrolling speed
• Configurable inter-loop pause duration
• Configurable separator
• Optional pause on hover or play on hover

Known issues

None so far.

Limitations and future considerations

It's opt-in so no breaking changes, but anyone wishing to use might need to update their CSS (implementation details leak a bit):

/* Before */
.volume .app-box > .title {
  ...
}

/* After: target both */
.volume .app-box > .title,
.volume .app-box > scrolledwindow > viewport > .title {
  ...
}

/* But 'any child' selector stays the same, works for both */
.volume .app-box .title {
  ...
}

For future extensions of other modules, even aside from just music (and its popup) I could see a lot of use-cases for making every Label scrollable like this. But this will necessarily mean replicating the same less-than-perfect UX pattern with regards to user CSS - wanted to hear your thoughts @JakeStanger on this.

[JakeStanger]

Thanks for this. I've raised a few bits in the first pass - there's quite a lot going on here, so I imagine this'll need a few runs through before I catch everything.

Agreed this will be a good thing to be able to build into the label module and custom widget, as well as introduce into other modules where it makes sense.

RE the styling concern, I think it's a non-issue. I've addressed it in a comment against the documentation.

[JakeStanger]

LGTM, cheers