[GREENFOOT-636] Convert about dialog to JavaFX

Currently, Greenfoot has its own Swing about dialog in greenfoot.gui.GreenfootAboutDialog. Since we would need to rewrite this into JavaFX, we may as well just delete it, and change bluej.pkgmgr.AboutBlueJ to be a shared AboutDialog class which takes the appropriate parameters for software title, website URL, and image name. I think even though they have some differences, parameterising is still better than maintaining two separate about dialogs.

(Something to perhaps discuss among the group at the time: Amjad and I did wonder in the past about having a fold-out section in the dialog showing previous team members, and translators, as a way to reward the translators. This should be fairly easy using JavaFX's TitledPane if we decide we want it.)

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Issue metadata

• Issue type: Task
• Priority: Medium