New plugin features to support JavaFX 11

[Don-vip]

With the removal of JavaFX in Java 11 (#16047), the following features won't work anymore by default:

• core: MP3 support
• plugin StreetSide
• plugin matsim

I don't want to include openjfx directly in JOSM core just for MP3 support. A better way could be to include it in a plugin on which streetside and matsim plugin can depend.

Problems to resolve:

• JavaFX is based on native libraries, the plugin must install only those for the current platform. If I remember correctly the JOGL plugin does already something like this.
• the plugin classes will have to be made available to JOSM core classloader for MP3 support => a new plugin property must be added for this (Core-Export?)
• the plugin will include OpenJFX 11. As we still need to support Java 8, the plugin must not be loaded on Java <= 10, for compatibility reasons => a new plugin property must be added for this (Main-Java-Version?)