GtkApplication: Lack of optional components shouldn't warn

GtkApplication: Lack of optional components shouldn't warn

When running uninstalled tests with GtkApplication on an autobuilder with
a fake session bus, warnings will cause the tests to abort. The GNOME
session manager, the Xfce session manager, and the Inhibit portal are all
not needed for normal operation of GTK, so we should not log warnings if
they are not found.

As well as not being present on a fake session bus, it's also not
expected that they'll be present on all platforms.

https://bugzilla.gnome.org/show_bug.cgi?id=774784