Fixed all bugs and ui layout, added a new feature.

[joji2468ng]

• Fixed the bug that mirroring blend joints in default biped is not working
• Fixed the bug that if click mirror with a regular joint selected will break the joint structure connection
• Changed the .json extension to .gst
• Changed the joint color to yellow which is the original color of the gimmick joint
• Added a new feature to flip the support joint by 180 degrees
• Refactored the codebase
• Added undo decorator to each function
• Fixed a bug that launching a window creates another window
• Removed three radio buttons for the side of the gimmick creation.
• Implemented error handling
• Retrieved guide data from a rig node instead of a guide node