#63 Add sdk extension

[jperedadnr]

This PR fixes #63 adding an sdk extension, similar to the JavaFX Gradle plugin one, that is used to override the JavaFX artifacts that will be selected, instead of using the existing Maven mechanism (with systemPath), which is still valid.

[betanzos]

The problem with this solution is that not work with javafx:jlink.

Maybe is a better idea to include a specific sdk configuration option for javafx:run and javafx:jlink.

[abhinayagarwal]

I stumbled on this requirement today. This would be a great win for developers who are testing their application across separate JavaFX versions.

@betanzos Can you briefly explain what do you mean by specific sdk configuration.

[betanzos]

@betanzos Can you briefly explain what do you mean by specific sdk configuration.

I mean, the JavaFX sdk is intended for runtime but not for link-time. Because of this, I think is a better approach to use the sdk definition only with the JavaFXRunMojo and have an option called jmods to use with JavaFXJLinkMojo.