ETSConfig app name heuristics break for entry points

Currently ETSConfig looks at the directory name that contains the `__main__` module. See:

https://github.com/enthought/traits/blob/master/traits/etsconfig/etsconfig.py#L370

For apps run via a `gui_scripts` entry point, this resolves to the `bin` directory of the environment on posix-ish systems (and probably something equally inappropriate on Windows). This is used as the default directory name for use with the application home directory and similar, so name collisions are particularly bad.

Overriding is awkward, as you have to override the whole application home, which is heavily OS-dependent.

Proposed behaviour might be to have an "application_id" that can be overridden as needed, possibly coupled with smarter heuristics.