Support extensions for importing/exporting a file type

[Cameron White]

For Pinta 1.5, we should support extensions that provide new importers/exporters for file types.

- Provide an easy way to register a new file type (just like for registering tools/effects)
- Document and provide example code for how to write this type of extension.

[Cameron White (cameronwhite91)]

Implemented methods for registering/unregistering a new file type: https://github.com/PintaProject/Pinta/commit/06f7f898fbfacadc1dca809f8e72c11f24bf052c

[Cameron White (cameronwhite91)]

The parent window is now passed to file exporters, which will allow those extensions to properly create dialogs if necessary.

[grofaty (grofaty)]

Is this bug fixed? I see a commit on GitHub: https://github.com/PintaProject/Pinta/commit/a0d75b9adb958727d35c1fe5ae472df863b7d6c5

[Cameron White (cameronwhite91)]

No, there's still more work to be done (like providing an example file format extension).

[Cameron White (cameronwhite91)]

Done: https://github.com/PintaProject/Pinta/wiki/Writing-a-File-Format-Addin