As per commit ed7ea51fb7, reintroduce
the "Fade" functionality for filters, by incorporating it directly
into GimpFilterTool.
Add "mode" and "opacity" options to GimpOperationSettings, and add
a corresponding "Fade" expander to the GimpFilterTool dialog
allowing to control them.
Reintroduce the FADE layer-mode context, and use it to mark the
layer modes avaialable for fading.
Add a new GimpOperationSettings class, to be used as a base class
for all operation-config types. The class provides options common
to all operations (namely, the clipping mode, input region, and
color options), which were previously stored in GimpFilterOptions,
and were therefore bound to the filter tool, instead of being
stored as part of the operation settings; as a result, these
options would have no effect when reapplying a filter, or when
restoring a preset.
The GimpOperationSettings options do not affect the operation
node, but rather the associated GimpDrawableFilter object. The
class provides a gimp_operation_settings_sync_drawable_filter()
function, which applies the options to the filter.
Modify all custom and auto-generated operation-config types to
derive from GimpOperationSettings, and modify the GimpConfig
functions of the former to account for the GimpOperationSettings
properties, using a set of protected functions provided by the
class.
Ell