Allow nodes to be made unselectable in the editor

Created by: shartte

Added the ability to lock selection in the 3D viewport for nodes and their children, making them 'click-through'.

The use case for this is mostly to exclude background art assets from the selection tool so that the region select / click select are more user friendly when doing level editing. Currently they would constantly select MeshInstance nodes from the background, rather than the nodes representing characters and such in the foreground. Having this feature allows me to simply exclude the entire node that contains all background art from being selected.

Note: This lacks a proper icon currently, as I just reused the lock transform icon. If this feature is even something that would potentially be merged, I'll have to figure out a good icon for it :)