Rewrite MirthPlainDocument #473

MirthPlainDocument is a copy of Sun JDK source with a proprietary header and needs to be rewritten from scratch.

client/src/main/java/org/syntax/jedit/MirthPlainDocument.java is a copy of the JDK's javax.swing.text.PlainDocument, taken from Sun JDK 1.5 source. It needs to be rewritten from scratch.

This is not open source license. The current OpenJDK version (PlainDocument.java in jdk17u) is GPL-2.0 with Classpath Exception, and is also not usable in our project.