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.
