"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:
<!-- Describe what you did (step-by-step) so we can 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 <TAB> to move selection from one handle to adjacent one,
> - deselecting selected gradient stop with <esc>,
> 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),
> - ...
issue