Optimize icon_for_app_id to avoid reading all desktop files for each new toplevel

[ids1024]

#117 fixed the most glaring issue here, and 8a4d4b6 make sure it at least doesn't block the UI thread, but it's inefficient to iterate over all desktop files for each opened app. I believe desktop_info_for_app_ids was copied from another Cosmic component (perhaps freedesktop-desktop-entry could provide better helpers here, once a better solution is in place).

If we just want to find a desktop file for an app id, we can only open one desktop file (in the first search directory we find it in), but the StartupWMClass comparison requires iterating over all desktop files. Is that needed? Only for XWayland clients? (What does Gnome do, etc?) Maybe it could only try that if it doesn't find an app with that id? I assume even if it's needed, it's okay for that to take lower precedence vs file name app ids...

It could also cache if there are multiple windows with the same app ID. Or cache icon names for all apps, but it should re-check if applications is changed, at least for newly installed apps.