Added links to element_* under @Seealso.

For improved navigation in help, I have added links to element_line, element_rect and element_text to the @Seealso section.
I also added element_blank for completeness.

Author: stefanedwards

R/theme.r

@@ -177,6 +177,10 @@ print.theme <- function(x, ...) str(x)
 #' @seealso \code{\link{+.gg}}
 #' @seealso \code{\link{\%+replace\%}}
 #' @seealso \code{\link{rel}}
+#' @seealso \code{\link{element_blank}}
+#' @seealso \code{\link{element_line}}
+#' @seealso \code{\link{element_rect}}
+#' @seealso \code{\link{element_text}}
 #' @export
 #' @examples
 #' \donttest{