Very basic balloon support.

This is a huge hack, setting it manually and never resetting it. Just
displaying the value (no breakdown) etc.

I'm tempted to drop this functionality altogether as it is of limited
use when you have the locals and watch windows.

python3/vimspector/stack_trace.py

@@ -34,6 +34,10 @@ class StackTraceView( object ):
     with utils.ModifiableScratchBuffer( self._buf ):
       self._buf[:] = None
 
+  def ConnectionClosed( self ):
+    self.Clear()
+    self._connection = None
+
   def LoadStackTrace( self, thread_id ):
     self._connection.DoRequest( self._PrintStackTrace, {
       'command': 'stackTrace',