terminology update: replace all instances of "Argument" with "Dynamic filter" in panels UI

It's terribly confusing for people new to Drupal to get that "contextual filters" in the views UI are the same thing as "arguments" in the Panels UI. It would be great to decide on a single term for that (I dislike the name "contextual filters", but that's another issue,see #1213078: Revisit language: is "Contextual filters" the right thing to call "Arguments"?) and use it consistently :)