Fix file chooser dialog on recipe export

[cydanil]

Description

This fixes #346 where it's described how recipes cannot be exported.
This was due to a bug in the interface between the file selector window, which returned only a list of filename(s), and the logic later that was expecting an extension to look up the correct plugin.

Now, the extension is directly provided, looked-up from the filetype combobox in the file chooser dialog.

How Has This Been Tested?

This was tested by:

1. Choosing a filename and a type in the combo box;
2. Setting a custom extension, but verifying that the type is correctly inferred from the combobox;
   The combo box gets updated when typing another valid extension. That is, if recipe.invalid is provided,
   then the file type is inferred from the combo box.
   If the filename is set to recipe.txt, then the combo box gets updated automatically, and its value is used, as above.
3. Not setting a filename greys out the Ok button, which simplifies the input validation.
4. Cancelling an export is correctly handled by returning None, None to the export manager.
5. The same base class is used to import images. Images can correctly be added to a recipe.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[cydanil]

The CI generated wheel is available here: gourmet.zip

[cydanil]

The list here is explicitly unpacked to catch empty lists (if the export was cancelled) to correctly return Nones to the export manager, cleanly cancelling the action.