Code refactor.

- Got rid off unwanted state information.
Incremental status updates.

- When thread updates are received by ThreadEventHandler, it only sends
those incremental updates instead of refreshing the whole debug status
window.

- When a thread is being stepped through, the complete set of variables
  are sent over to VIM.

VIM commands: These are internal commands used only by VimClient.
It is not meant for users.
- :JavaDebugThreadViewUpdate: Updates thread window.
- : JavaDebugVariableViewUpdate: Updates variable window.
- :JavaDebugSessionTerminated: Cleans up debug status window once
  session terminates.
New command to get detail view of a variable.

- JavaVariableDetail: The toString value of the Java object is displayed on
the status line.
Bug fix for variable disappearing from variable window.

If the fold is opened first and then value expand command is given,
then the value gets removed. This is becasue VIM fold requires the fold
to be opened before manipulating the entries under it.