When first loading the form, select the site default editor or the default member editor.
Split the form in two for clarity.

One for setting the default editor that is used when the member has
not set a preference.

One for setting the editor in the preferences of each existing user.
We filter out the 'Use site default' editor as default option on Plone 4.
Clarify strategy on Plone 4.
Preparing release 1.5
Back to development: 1.6