Fix PNGs with incorrect color profile.

> convert plover/assets/connected.png plover/assets/connected.png
convert: iCCP: known incorrect sRGB profile `plover/assets/connected.png' @ warning/png.c/MagickPNGWarningHandler/1830.

> convert plover/assets/disconnected.png plover/assets/disconnected.png
convert: iCCP: known incorrect sRGB profile `plover/assets/disconnected.png' @ warning/png.c/MagickPNGWarningHandler/1830.