app-system: Improve StartupWMClass heuristics

Our StartUpWMClass heuristics use a StartupWMClass -> .desktop ID
mapping built from the list of all installed applications. In case
of multiple .desktop files setting the same StartupWMClass, we
currently simply pick the last one returned by g_app_info_get_all (),
which can be a bit surprising:
A window with WM_CLASS 'emacs', launched through a .desktop file
named 'emacs.desktop' with a StartupWMClass of 'emacs' maps to ...
'emacsclient.desktop'!
Make this case a bit less random by preferring the app info whose
ID matches the StartupWMClass.

https://bugzilla.gnome.org/show_bug.cgi?id=751541