Add Shader support to StyleBoxes.

[INdIEDeV016]

The Problem:

As of now, I think there is no way to add a shader directly to a StyleBox...

...instead of adding a material to Control node...

...which sometimes also affects other things which are not desired (such as the shader or the material getting applied onto the text of the Button node as well, which is undesired)...

Possible Solutions:

1. What I think is, if we could extend the functionality of StyleBoxEmpty to be able to add shaders directly as a property. This way, even if a user wants to use it as a "display nothing" StyleBox, it will serve the purpose as well as provide the functionality of shading.
2. Add a new StyleBoxShader resource extending from StyleBox which could serve the same purpose as above.

The ability to add shaders to StyleBox means that it could be reused with other Control nodes and thus also partly enable us to use animations in themes (See #5660).

[Calinou]

This is already being tracked in #3494.