Integrate online reverse image search

This makes use of our image processing capabilities in order to
turn arbitrary image files into normalized thumbnails,
upload them to a temporary host, and pass the resulting URI
to a search provider.

In future, fiv should ideally run the upload itself,
so that its status and any errors are obvious to the user,
as well as to get rid of the script's dependency on jq.

README.adoc

@@ -43,7 +43,9 @@ Build-only dependencies:
 Runtime dependencies:
  gtk+-3.0, glib>=2.64, pixman-1, shared-mime-info, libturbojpeg, libwebp +
 Optional dependencies: lcms2, LibRaw, librsvg-2.0, xcursor, libheif, libtiff,
- ExifTool, resvg (unstable API, needs to be requested explicitly)
+ ExifTool, resvg (unstable API, needs to be requested explicitly) +
+Runtime dependencies for reverse image search:
+ xdg-utils, cURL, jq
 
  $ git clone --recursive https://git.janouch.name/p/fiv.git
  $ meson builddir
@@ -55,7 +57,8 @@ direct installations via `ninja install`.  To test the program:
 
  $ meson devenv fiv
 
-The lossless JPEG cropper is intended to be invoked from a context menu.
+The lossless JPEG cropper and reverse image search are intended to be invoked
+from a context menu.
 
 Windows
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