Previously, we had ShellAppInfo, which contains fundamental
information about an application, and methods on ShellAppMonitor
to retrieve "live" information like the window list.
AppIcon ended up being used as the "App" class which was painful
for various reasons; among them that we need to handle window
list changes, and some consumers weren't ready for that.
Clean things up a bit by introducing a new ShellApp class in C,
which currently wraps a ShellAppInfo.
AppIcon then is more like the display actor for a ShellApp. Notably,
the ".windows" property moves out of it. The altTab code which
won't handle dynamic changes instead is changed to maintain a
cached version.
ShellAppMonitor gains some more methods related to ShellApp now.
In the future, we might consider changing ShellApp to be a GInterface,
which could be implemented by ShellDesktopFileApp, ShellWindowApp.
Then we could axe ShellAppInfo from the "public" API and it would
return to being an internal loss mitigation layer for GMenu.
https://bugzilla.gnome.org/show_bug.cgi?id=598227
