improve UI integration on Mac OS X

Note that these changes have only been tested on OS X 10.6 running
Java 1.5. They may not work on more recent versions.

First, the UNIX launchers now set the apple.laf.useScreenMenuBar system
property to true, which enables the use of the system menu bar on Mac
and does nothing on other platforms.

Second, some rather tricky reflection code prevents the Dock icon from
turning into a coffee-cup icon once JESstartup initializes. I'm not
sure why it works, but it does (at least on barracuda).