Handle one dropped url by calling the appropriate handler.
The handler is first located by looking at `dnd-protocol-alist'.
If no match is found here, and the value of `browse-url-browser-function'
is a pair of (REGEXP . FUNCTION), those regexps are tried for a match.
If no match is found, just call `dnd-insert-text'.
WINDOW is where the drop happened, ACTION is the action for the drop,
URL is what has been dropped. Returns ACTION.