Dragging a nested widget to the outside of the parent widget is possible

[Tsvetelin Novkirishki]

Descriptive summary: User is able to drag a nested widget outside of the parent, which breaks is there is a relation between child and parent (there should be in most cases).

Steps to reproduce:

1. Insert some widgets inside of another widget
2. Select (with the mouse) a few fields
3. Drag them (not by the handles, just by the selection) to them outside of the parent
4. Do the same, but drag one by the handle
5. In all cases, it's possible to drag it to the outside

Browser and OS: Chrome and Firefox under Windows

[Piotrek Koszuliński]

This is a desired behaviour and we cannot block it by default. Imagine e.g. slideshow builder, when slides can be nested. One may want to rearrange them totally by grouping or ungrouping some slides.

An architecture allowing to control where content may be dropped will be proposed in #12091.