Editor: Visualise if images are used by layers

If images aren't used but loaded in the editor (external or embedded) the button will be drawn a bit darker than usual.

(Originally by deen, I have his permission to open this pull request (: )
That's not making it darker, it's making it more transparent. This currently works as the background is some kind of gray, but it might not work forever.

I something like

    ButtonColor *= vec4(0.5f, 0.5f, 0.5f, 1.0f)

should work, assuming that ButtonColor is a color representated in RGBA.