Allow calling get_property() with an uninitialized GValue

We already have the GType with which the GValue should be initialized, so requiring an initialized GValue is not really necessary, and it actually complicates code that wraps GObject by requiring the retrieval of the GParamSpec in order to get the property type. Additionally, it introduces a mostly unnecessary g_value_rest().

We already changed g_object_getv() to allow passing uninitialized GValues, but this fell through the cracks.

Closes: #737 (closed)