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.