Main menu of javatest window contains several configuration-editor
related actions, such as "Edit Configuration" "New Configuration",
which shows javatest configuration editor UI.

There should be a way to provide a custom made testsuite-specific
configuration editor UI and bound it to these actions.

The problem with the current javatest implementation is that
the action bindings are hard-coded and testsuite-specific
hooks that allows replacing javatest configuration editor
UI cannot be installed.

----- Proposed solution -------------

1. Exctract interface of class com.sun.javatest.exec.ConfigEditor
2. Class com.sun.javatest.exec.ConfigHandler should use the extracted
   interface to declare configEditor field.
3. Initialization of ConfigEditor made in ConfigHandler should be
   replaced
   with more flexible mechanism that uses TestSuite properties or
   InterviewParameters to create custom implementation of ConfigEditor.

The attached patch contains all modifications needed.