Add a check and warning if Jalview is compiled for Java 8 but being run by Java 11

Jalview compiled for java 8 (either the getdown/install4j installation or the executable jar) will run in java 11, but without a working web services client (due to certain libraries becoming external to the JVM).

Adding the JAVA_VERSION Jalview is compiled to into the .build_properties means that Jalview can check whether the right JVM is being used, and issue a warning if it isn't.

A console (stderr) warning could be made in jalview.bin.Launcher and jalview.bin.Jalview, and a popup could be made in jalview.bin.Desktop.

Gradle will need to be adjusted to add the JAVA_VERSION to the .build_properties file.