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-configure.sh

@@ -0,0 +1,25 @@
+#!/bin/sh -e
+# macos-configure.sh: set up a Homebrew-based macOS build
+#
+# Meson has no special support for macOS application bundles whatsoever.
+#
+# gtk-mac-bundler doesn't do anything particularly miraculous,
+# and it doesn't directly support Homebrew.
+#
+# It would be cleaner and more reproducible to set up a special HOMEBREW_PREFIX,
+# though right now we're happy to build an app bundle at all.
+#
+# It would also allow us to make a custom Little CMS build that includes
+# the fast float plugin, which is a bit of a big deal.
+
+# TODO: exiftool (Perl is part of macOS, at least for now)
+HOMEBREW_NO_AUTO_UPDATE=1 HOMEBREW_ASK=1 brew install
+	coreutils meson pkgconf shared-mime-info adwaita-icon-theme \
+	gtk+3 jpeg-xl libavif libheif libraw librsvg little-cms2 webp
+
+sourcedir=$(grealpath "${2:-$(dirname "$0")}")
+builddir=$(grealpath "${1:-builddir}")
+appdir=$builddir/fiv.app
+meson setup --buildtype=debugoptimized --prefix="$appdir" \
+	--bindir=Contents/MacOS --libdir=Contents/Resources/lib \
+	--datadir=Contents/Resources/share "$builddir" "$sourcedir"