Preserve expand/collapse state of variables and watches

This is starting to get horribly hacky and needs refactoring into
proper classes. Expandable variables now have the following additional
magic properties:

- '_expanded': True (expanded), False (collapsed), none = no preference
- '_variables': Current of child variables
- '_old_variables': The previous state of the '_variables' list when
                    refreshing

Remember that '_result' (the magic property of a watch expression) is
also treated like an expandable variable, so also has these magic
parameters.

python3/vimspector/debug_session.py

@@ -304,7 +304,8 @@ class DebugSession( object ):
     self.SetCurrentFrame( None )
 
   def SetCurrentFrame( self, frame ):
-    ret = self._codeView.SetCurrentFrame( frame )
+    if not self._codeView.SetCurrentFrame( frame ):
+      return False
 
     if frame:
       self._variablesView.LoadScopes( frame )
@@ -313,7 +314,7 @@ class DebugSession( object ):
       self._stackTraceView.Clear()
       self._variablesView.Clear()
 
-    return ret
+    return True
 
   def _StartDebugAdapter( self ):
     self._logger.info( 'Starting debug adapter with: {0}'.format( json.dumps(