PICARD-2130: Refactor cover art options

[phw]

Summary

• This is a…
  • Bug fix
  • Feature addition
  • Refactoring
  • Minor / simple change (like a typo)
  • Other
• Describe this change in 1-2 sentences:

Problem

• JIRA ticket (optional): PICARD-2130

The cover art options are currently not quite clear and can be confusing:

• The option "Use the first image type as the filename." is placed on CAA options, but applies globally
• Same for the "Save only one front image as separate file"
• It is not clear how images not affected by "Use the first image type as the filename." are saved, as the image name configuration is done elsewhere
• Some cover options apply to saving images as files, some for embedding images, but it is not quite clear what does what
• Inconsistent wording between "Save only one front image as separate file" and "Only embed a front image", even though both are conceptually very similar (one for saving separate files, one for embedding)
• Add the placeholder text "cover" to the cover_image_filename input. If this input is empty Picard will fall back to use just "cover" as filename, and having this placeholder text helps

Solution

• Rename the option to caa_image_type_as_filename to image_type_as_filename and move it to the global Cover options
• Rename the option to caa_save_single_front_image to save_only_one_front_image and move it to the global Cover options. This is consistent with the similar embed_only_one_front_image option.
• Separate Cover options into those for embedding and those for saving as files. This makes it clear which option actually applies. Also unavailable options are inactive.
• If save_only_one_front_image is enabled deactivate the image_type_as_filename checkbox, as it no longer applies (no non-front images will be saved)
• Consistent wording: "Embed only a single front image" and "Save only a single front image as separate file"
• Add the placeholder text "cover" to the cover_image_filename input. If this input is empty Picard will fall back to use just "cover" as filename, and having this placeholder text helps

[phw]

And of course the "Cover Art Archive" option page is now much simpler and really about options related to CAA downloading:

[zas]

LGTM