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.\[extension\]"

CCBUG: 446728