Cleanup Handle, Speedup Rendering, and Remove Dependencies

• Remove dependency on simplegeneric and decorator
• Speed up rendering by using a back-buffer
• Deprecate Handle.x and Handle.y, use Handle.pos instead

Port to Python3 and PyGObject

• Change license from LGPL 2.0 to Apache 2.0
• Port to Python 3 with support for 3.5, 3.6, and 3.7
• Python < 2.7 is no longer supported
• Port from GTK+ 2 and PyGTK to GTK+ 3 with PyGObject
• Migrate tests from Nose to PyTest with Tox
• Reorganize project files
• Overhaul the README
• Add contributing guide and code of conduct
• Adopt Black as code formatter
• Add Continuous Integration with Travis and Read The Docs

Port to Python3 and PyGObject

• Port to Python 3 with support for 3.5, 3.6, and 3.7
• Python < 2.7 no longer supported
• Port from PyGTK to PyGObject