Add icon for the application

This is the provisional icon for Cartero. Add it to the README and
update the Meson script files so that it also installs a desktop file to
run Cartero from the application launcher when installed.

README.md

@@ -1,3 +1,5 @@
+<img src="data/icons/es.danirod.Cartero.svg" width="128" height="128">
+
 # Cartero
 
 Make HTTP requests and test APIs.

data/cartero.desktop.in

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Name=Cartero
+Comment=Make HTTP requests and test APIs.
+Type=Application
+Exec=cartero
+Terminal=false
+Categories=GNOME;GTK;Network;Development
+# Translators: Search terms to find this application. Do NOT translate or localize the semicolons! The list MUST also end with a semicolon!
+Keywords=Gnome;GTK;HTTP;REST;
+# Translators: Do NOT translate or transliterate this text (this is an icon file name)!
+Icon=@app_id@
+StartupNotify=true