In the Java front end, don't try to guess the puzzle rectangle's

In the Java front end, don't try to guess the puzzle rectangle's
width and height by assuming mirror symmetry within the containing
applet area. Instead, use the proper width and height as given back
by the C sizing function.

(In particular, this fixes a bug where the non-blanked puzzle area
appeared too tall by the height of the menu bar, probably as a
result of confusing PuzzleApplet.getHeight() with
PuzzlePanel.getHeight(). But the mirroring approach was conceptually
wrong anyway.)

[originally from svn r9022]