"Enable gradient editing" in preferences allows stop deletion, but not addition

Migrated from https://bugs.launchpad.net/inkscape/+bug/1579395, opened by @Moini

Steps to reproduce:

• open Inkscape
• create a gradient and add another stop in the center by Ctrl+Alt+Click with the gradient tool
• Make sure the Node tool has "Enable Gradient editing" set to true
• Enter the node tool
• Delete the center stop using Ctrl+Alt+Click
• Add a new stop using Ctrl+Alt+Click

What happened?

Nothing.

What should have happened?

A new stop should be added. If deleting works, adding should work as well. (Same for adding by double-clicking, which works in the Gradient tool)

Inkscape Version and Operating System:

• Inkscape Version: 1.1-dev (c990a67, 2020-04-09)
• Operating System: Windows 10
• Operating System version: 10.0.18362

Migration info

In the LP issue, there is a long discussion whether this is a regression or not. tl;dr: it's probably not.

I just noticed there might be interference with 'Select under disregarding groups' for the Alt+Ctrl+Click thing - at least when 'enable gradient editing' would be checked for use with the selection tool (no problem for node tool)

On-canvas actions which were/are limited to the gradient tool (new in 0.42):

• adding a new gradient,
• nudging selected gradient stop with arrow keys,
• using to move selection from one handle to adjacent one,
• deselecting selected gradient stop with , as well as features added later to the gradient tool:
• reversing gradient direction (Shift+R) (0.44),
• selecting multiple gradient stops, with keyboard (0.46),
• selecting multiple gradient stops, by dragging a selection frame (0.46),
• adding intermediate stops (0.46),
• simplifying selected stops with Ctrl+L (0.46),
• ...