fix(css): created lm_popin_black.png
fix(css): created lm_popin_black.png

Fixes #545. Created the missing lm_popin_black png. Image is exactly the
same as lm_popin_white.png, but black.

golden-layout/golden-layout,deepstreamIO/golden-layout,deepstreamIO/golden-layout,deepstreamIO/golden-layout,golden-layout/golden-layout,golden-layout/golden-layout