Consider moving "enable.example_*" modules into "enable.examples" submodule

[rahulporuri]

The following modules are used only by the examples so we should consider moving them into enable.examles submodule instead of letting them live in the top-level enable namespace.

• enable.example_application
• enable.example_canvas
• enable.example_support

[aaronayres35]

Note this may be a breaking change outside of enable, for instance in chaco there are multiple places where we do
from enable.example_support import DemoFrame, demo_main