Show documentation, Java code translation on REPL

Two new features:

1. :help <name>:
   Here name can be a function name, class, a type:

frege> :help Applicative
This will launch a new gui window with the documentation for Applicative.

2. :java:

frege> :java
This will launch a new gui window with the Java source code translated from the last compiled Frege code.