MyPaint, fix impurity: add hicolor_icon_theme wrapper to ensure MyPaint can start

MyPaint fails to find its own icons and refuses to start if another
icon theme is not available on the system. Running
gtk-update-icon-cache on MyPaint's share/icons/hicolor folder did not
help, and setting the XDG_DATA_DIRS to point to MyPaint's icons also
didn't work.

https://gna.org/bugs/?18460

Thanks to Kim Simmons for reporting this and helping to debug.