Remove imports from `traitsui.editors.__init__`

A long time ago, all editor factories were imported directly from `traitsui.editors`. Over a decade ago, the `traitsui.editors.api` module was added, but import from `traitsui.editors` was still kept for backwards compatibility.

However this causes problems with circular imports in a number of places (eg. see discussion here: https://github.com/enthought/traitsui/blob/9f4acb87865e331b566628b9e4ecc08bec61e8d7/traitsui/null/rgb_color_trait.py#L145-L151).

We should remove the imports from `traitsui.editors.__init__` and fix situations where we are working around circular imports.

This would technically be a breaking change, but the impact is like very low at this point.