Auto reload shader files when changed on disk

[MartinDelille]

For most of the resources, they are automatically reloaded upon external changes. However, this is not the case for shader code.

This PR fixes that so the change is autmatically reflected both in the shader editor and in the viewport.

[MartinDelille]

Would someone tell me if this could be merged ?

[kitbdev]

Shaders in the text Shader Editor are already reloaded when edited externally.
Is there an open issue for this, and do you have reproduction steps for it not happening?

[MartinDelille]

When I edit externally a shader, it is reloaded only if Godot editor is focused again. I believed at the time I wrote the PR it did. I remember it was the case for other kind of resource like image but I can't reproduce with the current master.

[kitbdev]

I don't think anything in the Editor reloads immediately before it gets focused, and I'm not sure when it would have done that.
Also I tested and this implementation doesn't change that, Shader files are still only reloaded on focus. The signal used doesn't emit immediately on file change, only when a scan happens.