Build an application bundle on macOS

This is far from done, but nonetheless constitutes a big improvement.

macOS application bundles are more or less necessary for:
 - showing a nice icon;
 - having spawned off instances actually be brought to the foreground;
 - file associations (yet files currently do not open properly);
 - having a reasonable method of distribution.

Also resolving a bunch of minor issues:
 - The context menu had duplicate items,
   and might needlessly end up with (null) labels.

macos-svg2icns.sh

@@ -0,0 +1,22 @@
+#!/bin/sh -e
+# macos-svg2icns.sh: convert an SVG to the macOS .icns format
+if [ $# -ne 2 ]
+then
+	echo >&2 "Usage: $0 INPUT.svg OUTPUT.icns"
+	exit 2
+fi
+
+svg=$1 icns=$2 tmpdir=$(mktemp -d)
+trap 'rm -rf "$tmpdir"' EXIT
+
+iconset="$tmpdir/$(basename "$icns" .icns).iconset"
+mkdir -p "$iconset"
+for size in 16 32 128 256 512
+do
+	size2x=$((size * 2))
+	rsvg-convert --output="$iconset/icon_${size}x${size}.png" \
+		--width=$size --height=$size "$svg"
+	rsvg-convert --output="$iconset/icon_${size}x${size}@2x.png" \
+		--width=$size2x --height=$size2x "$svg"
+done
+iconutil -c icns -o "$icns" "$iconset"