Added initial support for alpha channel (in combine, decompose and blend nodes). Textures with alpha channels can be exported using the material node albedo input.
Updated node editor with a simple editor for enumerated parameters.
Exporting a material will now create a SpatialMaterial. The Material graph node
now generates different textures when used as a Godot addon (metallic, roughness
and ambient occlusion are merged into a single texture).
Rendering code (that was in graph_edit and node_base scenes) is now in a single "renderer scene" attached to the main window.
