GApplication: add a "resource base path"

We don't use this for anything inside of GApplication yet, but Gtk is
about to start using it to find various bits of the application (such as
its menus, icons, etc.).

By default, we form the base path from the application ID to end up with
the familiar /org/example/app style.

https://bugzilla.gnome.org/show_bug.cgi?id=722092
gapplication tests: test resource base path

Run some cases to make sure resource base path is behaving as we expect
it to.

https://bugzilla.gnome.org/show_bug.cgi?id=722092