Let children of Container nodes be selectionable, just forbid moving/rotation/scale

[StraToN]

Godot version:
Current master, 3.1 alpha

OS/device including version:
Mageia 6, all OSes affected

Issue description:
Since the merge of #22329, all children nodes of Containers are not selectable in the scene. Thus the only way to select these nodes is to search for them in the scene tree, which can be cumbersome.

While I understand the problems underlying described in #20750, I think it would have been interesting to keep the selection by mouse click in the scene, and simply lock all their geometrical parameters (position, size, rotation, scale...)
@groud @dualmatrix @reduz

Steps to reproduce:
N/A

Minimal reproduction project:
N/A