[GREENFOOT-638] Convert new-class dialog to JavaFX

Most classes in Greenfoot are created by right-clicking Actor/World and selecting new subclass. However, there is also a menu option Edit->New Class... which shows a simple dialog for creating other classes. This is in greenfoot.gui.NewClassDialog and should get converted to JavaFX (into .guifx.). You may want to use DialogPaneAnimateError as part of this (see e.g. BlueJ's bluej.pkgmgr.NewClassDialog, but that dialog is different enough that we don't want to re-use the whole dialog).

As part of this change, you will also need to convert the surrounding uses of the dialog. They currently use GPackage/GClass to create the class. This should be switched to using Package and co -- see use of BlueJ's NewClassDialog in PkgMgrFrame for some relevant code.

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Issue metadata

• Issue type: Task
• Priority: Medium