Remove imports from `traitsui.editors.__init__`

[corranwebster]

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:

traitsui/traitsui/null/rgb_color_trait.py

Lines 145 to 151 in 9f4acb8

	### Note: Declare the editor to be a function which returns the RGBColorEditor
	# class from traits ui to avoid circular import issues. For backwards
	# compatibility with previous Traits versions, the 'editors' folder in Traits
	# project declares 'from api import *' in its __init__.py. The 'api' in turn
	# can contain classes that have a RGBColor trait which lead to this file getting
	# imported. This will lead to a circular import when declaring a RGBColor
	# trait.

).

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.