Project icons

When you create a new project in Guppy, it asks you to select an icon.

Once the project is created, though, there is no way to update that icon.

For imported projects, it's even worse; you just get a random color and the first letter of the project name.

Ideally you should be able to change icons whenever, as well as upload your own, or choose to automatically use the favicon for the project (especially for imported ones).

This issue should be done, ideally, after #28, so that the new icon functionality can be used in both places.

This issue encloses several tasks:

• Add an "UploadProjectIcon component to select an image
• Copy that image to a new .guppy directory in the project (I think it has to live in the same directory as the project..?)
• Add a way to select the project favicon instead
• Add these new functions to the new-project wizard
• Add these new functions to the configuration pane in #28