Implement blur in Gala & Granite (now that Mutter & GTK do) and also more shell components (like popovers, dialogs, context menus etc) + Redesign

[azizkoumenji]

Now that Mutter and GTK have implemented blur, see: https://gitlab.gnome.org/GNOME/mutter/-/work_items/3023, https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/5071, https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/10145, https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/5160. I think Blur can now be implemented in Gala & Granite, since most of the work is already done on the Mutter and GTK side.

I think using more blur will give elementaryOS a fresher look. Having blurred sidebars, dialogs and context menus on apps for example and also adding more blur to shell components like the panel popovers, desktop context menu, dialogs etc... I know there's some opposition due to legibility and accesibilty concers (also blur is being seen more like just an esthetic prefrence) but the two major OSes (Windows and macOS) and linux desktop environments and window managers (like KDE Plasma, Cosmic, Hyprland etc) already implement blur without having those problem plus there could be a setting to disable blur OS wide to adress these issues.

The HIG will need to be updated in this case and also maybe Granite's surfaces (more flat) and icons in general will need to be redesigned so they look good with blur (maybe using a more mordern take on skeumorphic design like macOS Catalina and Bigsur icons for example) I feel like current icons are stating to look outdated, and especialy when used along with blur (again, this is mostly an aesthetic preference I know).

I'm not sure if this is a WIP for OS 9 and Granite 9, but I'm opening this discussion to get current opinions. I only found one discussion open about the app sidebar transparency.

Maybe adding these will attract more users since many of us chose elementary OS for its polished look.

@danirabbit