Fix install path of icons on Linux

[WOnder93]

The current hack with explicit "install" invocation is broken because it doesn't respect the $(INSTALL_ROOT) variable. Avoid manual installation of the icons by naming all of them "chessx.png" and placing them into individual subdirectories according to the size.

Unfortunately the native Qt install targets do not provide a way to rename the files when installing, so this is the only way to avoid running unix commands by hand.