Fix PNGs with incorrect color profile.

Recent libpng versions will output a warning when loading some of Plover PNG files, resulting in a warning dialog box everytime I start Plover. This merge request fixes the two offending files:

> 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.