Since commit 770231, StImageContent implements the GIcon interface, which
allowed us to represent all application icons as GIcon (app-info, X11 icon
property or themed fallback icon).
While that change made for a nicer ShellApp API, it did introduce a
conceptual issue in st_texture_cache_bind_cairo_surface_property():
GIcons usually represent static icons, while the ClutterContent
returned by that method updates automatically when the bound property
changes.
Address this by tracking the MetaWindow:icon property in ShellApp, and
update the fallback icon when it changes. With that, a GIcon object
always represents the same icon, and any icon change is reflected
by a corresponding GIcon change.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1761>
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