Implement downloading shaders from ShaderToy and GLSLSandbox and other shader-sharing sites

[KOLANICH]

I have tried a couple of shaders and saw them working. So I think, even though not all the shaders would work, having a feature to download and try shaders right in the app would be immediately useful. https://www.shadertoy.com/api http://glslsandbox.com/

[rafaellago]

I think that this may be a duplicate of #39

[KOLANICH]

Not quite, that one is mainly about compatibility, this one is mainly about having a builtin downloader.

[markusfisch]

Having a builtin downloader for those sites would sure be cool. Will come.