Get icons working correctly under Windows

With this we are able to include both a separate .ico file that the
program can load at runtime and a .res file (that is created from the .rc
file, both in the packaging/windows directory) that is linked into the
executable and makes the Windows Explorer show the correct icon for
subsurface.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>