Deprecate legacy demo module in extras

#940 moves the content of `traitsui.extras.demo` to another distribution. However other ETS projects are still importing from this module for showing their demo files (e.g. [`traits`](https://github.com/enthought/traits/blob/a48ede5e2b281f90bfe35b0a393c9d9cd7a77b11/examples/tutorials/tutor.py#L22), [`chaco`](https://github.com/enthought/chaco/blob/192675d6cb579df56458b87988026d895e7a7a84/examples/tutorials/tutor.py#L21)). Therefore a copy of the file is retained for the time being.

Once this separate demo distribution is released, projects should be contributing their demo examples to the application via entry points instead of using the `demo` function from `traitsui.extras.demo`. This module is not in the public API and is likely only used by ETS projects in their source's `examples` folders. However it would be good to deprecate the module first (this issue). A separate issue should be opened for removing it once other ETS projects have migrated from it.