Status command to view current threads and variables.

:JavaDebugStatus command will open a temp window in the bottom and
vertically split it. One one side it will show the threads and another
side it will show the variables of the currently suspended thread.

This is just an initial checkin. Needs more display formatting.

Testing:
Verified the command shows threads and variables.
Enhancements to debug status view.

* Display status when :JavaDebugStart command is run. It does not auto
  refresh yet.
* util.vim: Enable creation of temp window with vertical split. There
  are some issues in this code, so its not used by debug.vim in this
  change. So the thread view and variable view are shown as horizontal
  splits. This will be fixed in next CL.

Testing:
Status window shows correctly after starting a debug session.
Test code.

* Simulate multiple threads to test stackframes.