[GREENFOOT-257] Make it possible to remove MP3 library without breaking greenfoot

In order to be able to package Greenfoot for some linux distribution, it should be possible to exclude MP3 support by just deleting the jlayer library.

When attempting to play back an MP3, it should show an error message with instructions on how to get MP3 support back.

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Issue metadata

• Issue type: Bug
• Priority: Medium
• Fix versions: 1.5.5