To be used by the blend tool gradient editor to edit the gradient
endpoint/stop/midpoint properties corresponding to the selected
handle.
The GUI is currently empty; the following commits add its contents.
When a midpoint is double-clicked, convert it into a gradient stop
(i.e., split the corresponding segment at the midpoint,) by
responding to the line's handle-clicked signal.
Add a tentative_gradient member to GimpBlendTool, which, when set,
is displayed instead of the current gradient.
Use this to show a version of the gradient with the currently
selected stop deleted, upon receiving a prepare-to-remove-slider
signal, i.e., when the slider is about to be removed.
Add a boolean "modify active gradient" option to the blend tool.
when checked, the active gradient is modified in-place while edited.
When unchecked, the active gradient is copied to the internal
"custom" gradient upon editing, and the custom gradient becomes
subsequently active.
Show a hint when the option is checked, but the active gradient is
non-writable, and can't be edited directly.
This commit adds the new gimpblendtool-editor.[hc] files, which are
where the gradient-editing related functionality of the blend tool
is going to go.
