fix(image): symbolic icons not rendering in GTK 4.20+

[slowsage]

For symbolic icons to work with ironbar's icon_theme, we need to adapt to GTK 4.20 changes.

Required more refactoring than intended to

1. keep the Image vs Picture check localized
2. handle both sync (Image) and async (Picture).

Also removed duplicate feature = "clipboard",.

Relevant:
https://docs.gtk.org/gtk4/migrating-3to4.html#adapt-to-gtkicontheme-api-changes
https://docs.gtk.org/gtk4/migrating-3to4.html#dont-use-gtk-icon-theme-in-your-css

Fixes: #1039

Given the scope, prolly makes sense for v0.19.

[JakeStanger]

Code lgtm, thanks.

What impact this has on GTK versions prior to 4.20? I would assume nothing but I'd like to confirm that prior to merge.

Did the GTK4 port cause a regression in the behaviour in any way, or is this fixing the issue as it was in GTK3?

[slowsage]

What impact this has on GTK versions prior to 4.20? I would assume nothing but I'd like to confirm that prior to merge.

My understanding it it should work on gtk 4 prior to 4.20 - simply avoided is_symbolic (carried over from 3.0) and get_icon_name (added in 4.0) - both got removed in 4.20.

Did the GTK4 port cause a regression in the behaviour in any way, or is this fixing the issue as it was in GTK3?
AFAIK, the issue was there in both versions:

• GTK3 - ironbar should have used set_from_icon_name for symbolic icons - do not think it did.
• Even if it did, it would not have respected icon_theme.
• The proper icon_theme respecting solution is only possible in gtk4 AFAIK.

Edit: In gtk3, there was a css based gtk-icon-theme config so it may have been possible to inject icon_theme into that to set theme even on a per-module basis. But per this: GTK 4 always uses the current icon theme, with no way to change this.

[JakeStanger]

Thanks for clarifying. I'll give it a quick test to make sure, but otherwise sounds like targeting 0.19 is sensible.