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.

README.adoc

@@ -2,7 +2,7 @@ fiv
 ===
 
 'fiv' is a slightly unconventional, general-purpose image browser and viewer
-for Linux and Windows (macOS still has major issues).
+for Linux and Windows (macOS also kind of works).
 
 image::docs/fiv.webp["Screenshot of both the browser and the viewer"]
 
@@ -91,6 +91,14 @@ _mingw-w64-lcms2_ with the following change:
 
  sed -i 's/meson setup /&-Dfastfloat=true /' PKGCONFIG
 
+macOS
+~~~~~
+Support for this operating system isn't as good.
+If you install Homebrew, you can get an application bundle with:
+
+ $ sh -e macos-configure.sh builddir
+ $ meson install -C builddir
+
 Documentation
 -------------
 For information concerning usage, refer to link:docs/fiv.html[the user guide],