app: Add UI for adjusting the gradient points in GimpBlendTool
app: Remove last_{x,y} from struct _GimpBlendTool

and use local variables instead.
app: Relax preconditions on gimp_gradient_get_color_at

The context can now be null unless the gradient specifically needs
the foreground/background colors, in which case
gimp_context_get_{foreground,background} will do the check and warn.
app: Add a gegl op to render gradients
app: Prevent grabbing handles after halting in the blend tool.
app: Add a basic live preview to GimpBlendTool using GimpImageMap
app: In GimpBlendTool, sync the operation properties with GimpBlendOptions
app: Sync opacity and paint-mode properties with blend_tool->image_map
app: When constraining angles, don't allow points to get out of line.
app: Update the gradient tool preview when the gradient is modified.
app: Support gradients with fg/bg segments in the blend tool preview.
app: Redo the handles in GimpBlendTool