Proper DMG support on macOS.

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.

custom/macos-mkdmg.sh

@@ -0,0 +1,45 @@
+#!/bin/sh
+
+BUNDLE=TuxPaint.app
+TEMP_DMG=temp.dmg
+TEMP_DMG_SIZE=`expr \`du -sm "$BUNDLE" | cut -f1\` \* 15 / 10`m
+FINAL_DMG=TuxPaint.dmg
+VOLNAME="Tux Paint"
+ICON="macos/tuxpaint.icns"
+BACKGROUND="macos/background.png"
+
+
+echo "   * Creating the temporary image..."
+hdiutil create "$TEMP_DMG" -ov -fs HFS+ -size "$TEMP_DMG_SIZE" -volname "$VOLNAME" \
+&& VOLUME=`hdiutil attach "$TEMP_DMG" -nobrowse -noverify -noautoopen | grep Apple_HFS | sed 's/^.*Apple_HFS[[:blank:]]*//'` \
+|| exit 1
+
+echo "   * Adding the image background..."
+mkdir "$VOLUME/.background" \
+&& tiffutil -cathidpicheck "$BACKGROUND" -out "$VOLUME/.background/background.tiff" \
+|| exit 1
+
+echo "   * Setting the folder icon..."
+cp "$ICON" "$VOLUME/.VolumeIcon.icns" \
+&& xattr -wx com.apple.FinderInfo '00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00' "$VOLUME" \
+|| exit 1
+
+echo "   * Copying the contents..."
+ln -s "/Applications" "$VOLUME" \
+&& cp -a "$BUNDLE" "$VOLUME" \
+&& cp -a "macos/DS_Store" "$VOLUME/.DS_Store" \
+|| exit 1
+
+echo "   * Configuring the folder to open upon mount..."
+bless --folder "$VOLUME" --openfolder "$VOLUME" \
+|| exit 1
+
+echo "   * Unmounting the temporary image..."
+hdiutil detach "$VOLUME"
+
+echo "   * Creating the final image..."
+hdiutil convert "$TEMP_DMG" -ov -format "UDBZ" -imagekey "zlib-level=9" -o "$FINAL_DMG"
+
+echo "   * Deleting the temporary image..."
+rm -f "$TEMP_DMG"
+