More gradient options (path, shape,radial2)

Migrated from https://bugs.launchpad.net/inkscape/+bug/170421 🍷

The Idea:

Additional Gradient options:

• Gradient along path
• Gradient along shape
• Gradient along radial (conical)
• invert/swap colors

Original text (edit):

...gradient path : same as linear but following a path,
^ shape : same as radial but following contour of shape
^ radial 2 : start with color 0 at 0.1 degree and end with color 1 at 359.1 degree

More useful:
X shape: for every line start from 0 in left shape-boundary and end to 1 in right shape-boundary <edge> ¹
Y shape: same as above but from top to bottom edge (this last one is good for an aurora borealis) ¹

For the gradients options: invert colors (0 swap with 1) in gradient edit or multi color gradient (red->yellow->white)

Additional Information/Comments/Clarifications:

• Another "Believe It Or Not" non-duplicate issue here on GitLab...and if I'm wrong I owe you a beer. 🍻
• Gradient along shape:
  • GIMP's Gradient tool: Shaped (Angular/Spherical/Dimple)
• Gradient along radial:
  • NeXTSTEP/OPENSTEP/OSX/macOS "Spinning Beachball of Death" 😆
  • Logos by Nick has a tutorial on how to achieve this since 0.92.1...but respectfully, it's a hack.
• Invert/Swap Colors:
  • Blender's Color Ramp node: Color > Color Ramp > Flip Color Ramp
  • GIMP's Gradient tool: Reverse
• Tangential: IIRC Creature House Expression had similar gradient abilities. To this day nobody has come close replicating certain features that application had...but I digress.

Footnotes:

¹ Variations on Gradient along path?