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.
ed7ea51fb7