ShellAppSystem: own the memory for the startup wm class and app id

The hash table must keep a copy of the IDs, because the GAppInfos
are unreferenced (and thus freed) at the end of the function.
This was possibly not a problem if the GAppInfos were referencing
the memory-mapped cache, but it becomes one for regularly parsed
desktop files in ~/.local.

https://bugzilla.gnome.org/show_bug.cgi?id=721039