Drag and drop image to visual shader should create constant texture node immediately

[ZX-WT]

Godot version:

3.1 alpha

OS/device including version:

nil

Issue description:

I am playing with the visual shader editor and now I can drag and drop the image into a node after I create the constant texture node, it will be more direct if one can just drag and drop the image into the editor and the node will be created automatically. As in most case, I believe that's how a texture will be used in a shader.

Steps to reproduce:

Minimal reproduction project:

[lentsius-bark]

Mate, this is a brilliant idea!