Improve ImageBrowserDialog

[cydanil]

Description

This PR improves gourmet.image_utils.ImageBrowser by making the image retrieval a background thread, thus not blocking until all images are retrieved, and making the dialog scrollable.

How Has This Been Tested?

This was tested by importing a web recipe.

Screenshots (if appropriate):

The dialog used to expand each time an image was added, often making the dialog expand beyond the screen:

It now retains its size:

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.