explicit `toFront` call for `show-window`

When `show` is called, the graph does not appear at the front.

A `java.awt.Window.toFront()` call needs to be made in order to get the window to appear.

https://github.com/Clojure2D/clojure2d/blob/master/src/clojure2d/core.clj#L2102