Let the file dialog open in the EDT.

Without that statement the save action sometimes freezes on Mac OS X and GreenFoot 2.4.
There are some more dialog actions in this scenario which also need to be checked if they run on or off the EDT.
Update the greenfoot API to ensure we're calling the right methods from Greenfoot
Ensure all popup dialogs (messages, file chooser, etc.) open in the Event Dispatcher Thread so it doesn't freeze the UI (see Issue #1).
Specify last instantiated world for the Sokoban projects and improve warnings
Prepare release 2.1.1
Merge branch 'uhafner-edt'