Add support for classes in JRT

[helospark]

What

• When the class is located in the JRT filesystem, extract content from there, instead of treating it as a jar
• PR for can not decompile class javax.crypto.Cipher in jdk10 with FernFlower #55

Why

• Java 9 no longer contains it's class files in JAR files like previous versions, it now uses a modules file. https://openjdk.java.net/jeps/220
• So trying to load a JDK class from jar it will fail to decompile. Most JDK classes have source in the JDK src.zip, so it won't affect those

How

• Similarly to how Eclipse does internally

Notes

• JRTUtil was introduced in Eclipse Oxygen, so I had to raise the version.
  However this should not limit the installability of the plugin, because the jdt versions are not fixed in MANIFEST file. I have added an additional catch, so earlier versions of Eclipse will not fail with ClassDefNotFound exception.
  When the baseline of the plugin is incremented, that try-catch can be removed

[pbi-qfs]

Sounds great, thank you!

[RobertZenz]

I'll be unable to review this in depth for various reasons within the next time. Can somebody else please test this?