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,
| pspec |
A ParamSpec declaring the metadata value |
| value |
A Value containing the value to parse. |
|
true if parsing is successful. |