Previously, the DMG's default window is slightly larger than the
background, exposing some whitespace. This fixes the window size to be
identical to the background image, so no whitespace is exposed.
Previously `make install` on macOS created TuxPaint.dmg, but it was a
simple disk image.
Now TuxPaint.dmg is formatted as one would expect from any other macOS app,
with a Tux folder icon, symlink to the /Applications folder, and a
background image with an arrow.
