Take fields into account during text conversion

* lisp/cus-edit.el (Custom-mode): Enable text conversion, now
that fields are correctly treated.

* src/alloc.c (mark_frame): Mark f->conversion.field.

* src/androidterm.c (android_update_selection): Adjust
conversion region and selection position by the field start and
end.

* src/editfns.c (find_field): Export function.

* src/frame.c (make_frame): Clear f->conversion.field.

* src/frame.h (struct text_conversion_state) <field>: New field.

* src/lisp.h (find_fields, reset_frame_conversion): Export
functions.

* src/minibuf.c (Fread_from_minibuffer): Reset frame conversion
if Voverriding_text_conversion_style is set.

* src/textconv.c (textconv_query): Narrow to field.
(reset_frame_conversion): New function.
(reset_frame_state): Clear conversion field.
(really_delete_surrounding_text): Narrow to field.
(locate_and_save_position_in_field): New function.
(really_request_point_update, really_set_point_and_mark)
(complete_edit_check, handle_pending_conversion_events_1)
(handle_pending_conversion_events, get_conversion_field)
(set_composing_region, textconv_set_point_and_mark, replace_text)
(get_extracted_text, get_surrounding_text, report_point_change):
Compute, narrow to and offset by the currently active field
whenever point is updated or a command is received.
(syms_of_textconv): Revise doc strings.

* src/textconv.h (get_conversion_field): Export function.

lisp/cus-edit.el

@@ -5400,6 +5400,7 @@ if that value is non-nil."
   (setq-local custom--invocation-options nil
               custom--hidden-state 'hidden)
   (setq-local revert-buffer-function #'custom--revert-buffer)
+  (setq-local text-conversion-style 'action)
   (make-local-variable 'custom-options)
   (make-local-variable 'custom-local-buffer)
   (custom--initialize-widget-variables)