Disallow Nesting Horizontal containers inside each other via dragging

[grtlr]

Describe the bug

Nesting Horizontal or Vertical containers should be disallowed in the UI. This works for the Add button by greying out these options. However, when dragging a horizontal container inside another horizontal container it simply disappears without user feedback.

As per @Wumpf, this might be related to the automatic cleanup of containers being to aggresive.

To Reproduce

Screen.Recording.2024-11-14.at.15.47.36.mov

Expected behavior
The dragging target should be dissallowed, or the user should be informed about the cleanup.

Desktop (please complete the following information):

• OS: macOS Ventura 13.7

Rerun version
Pre-release release-0.20.0