Complete jpeg support

[cmbruns]

As first test of new "how to add a new image format" wiki page.

1. [x] Create a small image in the new format and place it in test/images
2. [x] Make sure the image loads correctly when you drag it onto vimage.
3. [x] Ascertain whether you can save this image format correctly in vimage.
4. [x] In main_window.py
   1. [x] Add the file extension to the list of extensions mentioned in VimageMainWindow.load_main_image()
   2. [x] If this format can be saved, add its extension to the list in VimageMainWindow._dialog_and_save_image()
5. [x] Create a new windows file association in file CMake/wix_patch.xml
   1. [x] Create a new stanza in the #PRODUCT section, similar to the JpgOpenAssociation stanza
   2. [x] Insert the correct extensions and mime types. Consult https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Image_types for some file type conventions.
   4. [x] Add a corresponding ComponentRef in the #ProductFeature section
   5. [x] (don't worry too much about the FileTypesFeature section until we get that part of the installer working...)
6. [x] Create a new MacOS file association in the file CMake/vimage_macos.spec
   1. [x] Under CFBundleDocumentTypes, add a new stanza for the new file type.
   2. [x] Consult https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html for some mac file type conventions.