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-Info.plist.in

@@ -0,0 +1,52 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+	<dict>
+		<key>CFBundleExecutable</key>
+		<string>@ProjectName@</string>
+		<key>CFBundleIdentifier</key>
+		<string>@ProjectNS@@ProjectName@</string>
+		<key>CFBundleName</key>
+		<string>@ProjectName@</string>
+		<key>CFBundleIconFile</key>
+		<string>@ProjectName@.icns</string>
+		<key>CFBundleShortVersionString</key>
+		<string>@ProjectVersion@</string>
+		<key>CFBundleInfoDictionaryVersion</key>
+		<string>6.0</string>
+		<key>CFBundlePackageType</key>
+		<string>APPL</string>
+
+		<!-- Although mostly static, this should eventually be generated. -->
+		<!-- In particular, we should expand image/x-dcraw, -->
+		<!-- using information we can collect from shared-mime-info. -->
+		<key>CFBundleDocumentTypes</key>
+		<array>
+			<dict>
+				<key>CFBundleTypeName</key>
+				<string>Image File</string>
+				<key>CFBundleTypeRole</key>
+				<string>Viewer</string>
+				<key>LSHandlerRank</key>
+				<string>Default</string>
+				<key>LSItemContentTypes</key>
+				<array>
+					<string>com.apple.icns</string>
+					<string>com.apple.quicktime-image</string>
+					<string>com.compuserve.gif</string>
+					<string>com.microsoft.bmp</string>
+					<string>com.microsoft.ico</string>
+					<string>org.webmproject.webp</string>
+					<string>public.avif</string>
+					<string>public.heic</string>
+					<string>public.heif</string>
+					<string>public.jpeg</string>
+					<string>public.png</string>
+					<string>public.svg-image</string>
+					<string>public.tiff</string>
+					<string>public.xbitmap-image</string>
+				</array>
+			</dict>
+		</array>
+	</dict>
+</plist>