setup_win32.py: Add initial separate setup for win32 build
build_helpers: Extend base build's subcommands instead of subclassing

This plays more nicely with cx_freeze's build step.
build_helpers: Place gschemas elsewhere if frozen

This still doesn't actually compile gschemas; for now that remains a
manual process on Windows.
build_helpers: Attempt at building translations, but still not working

intltool is the current blocker here, and it requires a more complete
build environment that we currently assume.
build_helpers: Install icons in share/meld when frozen
data/icons/meld.ico: New Windows icon file for main Meld icon
setup.py: Fix for distutils build change