Modified ExtendedPythonGenerator so it can safely be a singleton

[lizlooney]

Fixed #63

Added generator_context.ts with class GeneratorContext, which holds information needed during and after python code generation:

• The module (so the ExtendedPythonGenerator can get the class and parent class names)
• Exported blocks (these are produced by the ExtendedPythonGenerator and are added to the file after code generation is finished).
• The class method names (maps from the block's NAME field to the actual python method name)
• Method getClassName returns the class name as PascalCase.

Modified ExtendedPythonGenerator so it can safely be a singleton.

• Removed fields currentModule and mapWorkspaceIdToExportedBlocks and added context (type GeneratorContext).
• Renamed methods_ field to classMethods. This field can stay in ExtendedPythonGenerator because it is only needed during workspaceToCode execution. It is not needed after workspaceToCode has finished.
• Added a field name workspace for the Blockly Workspace because it is needed in the finish method, but it is not a parameter.
• Removed init method override.
• Added workspaceToCode overload with additional parameter context (type GeneratorContext).
• Renamed method addMethod to addClassMethodDefinition and added NAME field value as a parameter.
• Added finish method override. -- Here we still produce the code, which is the class definition line and the class method definitions. -- By setting the code parameter to the code we want (the class definition) and passing it to super.finish, we can just call super.finish normally. -- Produce the exported blocks.
• Moved setCurrentModule (renamed to setModule), getExportedBlocks, and classParentFromModuleType (renamed to getClassParent) to GeneratorContext.

Updated the App to create a GeneratorContext and pass it to workspaceToCode. Updated common_storage to create a GeneratorContext and pass it to workspaceToCode. Updated Editor to create a GeneratorContext and pass it to workspaceToCode.

Modified code generator for mrc_class_method_def blocks to return empty string.

[alan412]

LGTM and verified it works on my system