app: remove "Edit -> Fade..."

This commit completely removes the "Edit -> Fade..." feature,
because...

- The main reason is that "fade" requires us to keep two buffers,
  instead of one, for each fadeable undo step, doubling (or worse,
  since the extra buffer might have higher precision than the
  drawable) the space consumed by these steps.  This has notable
  impact when editing large images.  This overhead is incurred even
  when not actually using "fade", and since it seems to be very
  rarely used, this is too wasteful.

- "Fade" is broken in 2.10: when comitting a filter, we copy the
  cached parts of the result into the apply buffer.  However, the
  result cache sits after the mode node, while the apply buffer
  should contain the result of the filter *before* the mode node,
  which can lead to wrong results in the general case.

- The same behavior can be trivially achieved "manually", by
  duplicating the layer, editing the duplicate, and changing its
  opacity/mode.

- If we really want this feature, now that most filters are GEGL
  ops, it makes more sense to just add opacity/mode options to the
  filter tool, instead of having this be a separate step.

devel-docs/app/app-docs.sgml

@@ -1336,7 +1336,6 @@
       <xi:include href="xml/channel-options-dialog.xml" />
       <xi:include href="xml/convert-dialog.xml" />
       <xi:include href="xml/data-delete-dialog.xml" />
-      <xi:include href="xml/fade-dialog.xml" />
       <xi:include href="xml/file-open-dialog.xml" />
       <xi:include href="xml/file-open-location-dialog.xml" />
       <xi:include href="xml/file-save-dialog.xml" />

devel-docs/app/app-sections.txt

@@ -1068,7 +1068,6 @@ gimp_edit_named_copy
 gimp_edit_named_copy_visible
 gimp_edit_clear
 gimp_edit_fill
-gimp_edit_fade
 </SECTION>
 
 <SECTION>
@@ -11379,7 +11378,6 @@ gimp_image_undo_group_start
 gimp_image_undo_group_end
 gimp_image_undo_push
 gimp_image_undo_can_compress
-gimp_image_undo_get_fadeable
 </SECTION>
 
 <SECTION>
@@ -11779,11 +11777,6 @@ dialogs_get_toolbox
 about_dialog_create
 </SECTION>
 
-<SECTION>
-<FILE>fade-dialog</FILE>
-fade_dialog_new
-</SECTION>
-
 <SECTION>
 <FILE>image-new-dialog</FILE>
 image_new_dialog_new
@@ -13612,7 +13605,6 @@ edit_redo_cmd_callback
 edit_strong_redo_cmd_callback
 edit_strong_undo_cmd_callback
 edit_undo_clear_cmd_callback
-edit_fade_cmd_callback
 </SECTION>
 
 <SECTION>