Include the default value of a property in the GIR data
We use g_param_spec_get_default_value() to get the default GValue of a GParamSpec, and then we serialize it into a string according to the value's own contents and type.
The default value is stored inside the GIR data, as the conversion to a string is lossy by definition, and will rarely (if ever) round-trip.
To allow for property values that do not allow transformations to a string, we add a freeform default-value
annotation.
Fixes: #4 (closed)