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.
Updated all nodes so parameters are located in a "parameters" variable.
Started writing a generic node model whose behavior is defined in a json file. Should be able to replace all generators so far.
Wrote json files for bricks and noise nodes.
Started preparing a custom node.
Added a class for gradient (that handles serialization and shader generation), and updated the GradientEditor.
Added a "Types" class that handles values serialization.
Updated linked_control and config_control to link to GradientEditors.
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.
