Niels De Graef authored Aug 12, 2019 and Jehan committed Aug 16, 2019

GimpConfigWriter contains several constructors with the convention
`gimp_config_writer_new_* ()`. This will lead to problems however with
languages like Vala, where it cannot disambiguate the following:

```
// calls config_writer_new_string()
Gimp.ConfigWriter w = new ConfigWriter.string("xxx");
// calls config_writer_string()
w.string("xxx")
```

Using `from_` in constructors is general practice in GObject-bsed
libraries because of this.

This also fixes an error when trying to use vapigen on the GIMP .GIR
file.