app: Consider "new-window" action for opening new windows

While can_open_new_window() uses some elaborate heuristics to predict whether an application can open multiple windows, open_new_window() will always simply relaunch the application. This is often the best we can do, but when an application provides a "new-window" action in its .desktop file or on the bus, it is much more likely to work as expected than blindly activating the app and hoping for a particular behavior.

https://bugzilla.gnome.org/show_bug.cgi?id=756844

Marking as WIP as the change is still under discussion