Use a better and simpler default name for multi-file archives
Now the default name of a multi-file archive created from the file actions plugin is the common prefix of those file's filename or "Archive.[extension]", if they don't match, instead of "[name of parent folder].[extension]".
aka:
"Screenshot_123.jpg", "Screenshot_1234.jpg" => "Screenshot_123.[extension]"
"cats.jpg", "dogs.jpg" => "Archive.[extension]"
Example for multiple screenshot files:
cc @ngraham
superseeds !96 (closed)
Replaces !98 (closed)