gobject: Allow install_properties with 1 GParamSpec

g_object_install_properties() can be used to install multiple properties at once, and to allow an optimization for property notification. Arguably, using it over g_object_install_property() is best practice. That being said, you can't use it if you have only a single property that is installed (yet), due to a precondition check for more than 1 pspecs.