1.0dev: Fix ''Comments only'' checkbox:
* Keep UI state consistent on auto-preview. * Never hide ''new'' (possibly conflicting) comments, nor attachments. * Default to 'false' to avoid `False != ''false''` issue, as boolean preferences decay to strings in DB.
Resolves #10766.
Authored by: psuter 2012-07-17
Parent: [r11123]
Child: [r11125]
