Add ConfigSectionForm for managing a single INI section

Extends ConfigForm to work with one named section at a time: element
names map directly to keys within the section instead of using the
double-underscore convention. Supports creating new sections (when no
section name is passed), renaming sections (ON_RENAME), and deleting
sections (ON_DELETE). Deletion bypasses full form validation so broken
sections can always be removed. Empty sections are preserved on save
because their presence alone can indicate that a resource exists and
should use default values.

Author: TheSyscall