parse_value

If a signal handler is connected to `parse_value` a signal is emitted when the file module accesses a value using gegl_metadata_set_value.

The signal handler should parse the value supplied in the Value and may set any number of metadata values using set_value.

If no handler is connected the mapped metadata value is set normally,

Parameters:

pspec	A ParamSpec declaring the metadata value
value	A Value containing the value to parse.

Returns:

true if parsing is successful.