Support metadata scraping to multiple files

[ThePromidius]

The idea is:

• Have multiple files selected.
• Series name must be the same for all of them so the series field can't be "## Keep Original Value ##"
• The metadata will be applied to all of the selected files.
• Existing values will not be wiped if the source does not fill these values.
• Volume and number should keep original.

[majora2007]

I can start some of this by adding code to disable the fetch flow if multiple files are selected and they have different series names. I'm doing some cleanup in the files needed, so I will wire up the needed checks and process improvements.