Change scene drag-and-drop modifier behavior in the 2D and 3D editors

[Calinou]

Behavior is now consistent across 2D and 3D editors.

• By default, drag-and-dropping adds the scene as a child of the selected node. Hold Alt when releasing the mouse to add the scene as a child of the root node, or Shift to add the scene as a sibling of the selected node.
• To choose a different node type in the 2D editor when drag-and-dropping a texture resource, hold Alt + Shift (instead of just Alt).
• If multiple nodes are selected, only the first one is taken into account. This was previously the behavior in 2D, but not in 3D (an error dialog appeared instead). This makes the UI more forgiving.

Ctrl is no longer used in shortcuts because it interferes with the grid snapping toggle. Using Alt in shortcuts here is fine, because it's checked when the mouse is released, not when it's pressed. Therefore, it'll work correctly on Linux with window manager defaults.

---

• This closes Dragging an object does not respect selection. #80795.

Preview

I hold Alt when performing the last drag-and-drop operation, which is why the last node is added to the root node instead.

simplescreenrecorder-2024-01-26_18.00.20.mp4

[ryevdokimov]

#87126 is related.

[ryevdokimov]

Tested every scenario in the description. Seems to work.

[KoBeWi]

For 2D, the current behavior was implemented in #51011
Although before that there was no way to control where the node is added, so changing the default is fine.

[KoBeWi]

Using Alt+Shift for changing type makes it impossible to change type and add as sibling.

[KoBeWi]

Looks fine overall.

The Shift+Alt conflict I mentioned is a corner-case, I'm not sure what modifier we could use to deconflict it. Maybe it's fine this way.

[akien-mga]

Thanks!