Add .desktop file

This adds a .desktop file (without icon for now), which will automatically be installed when
`meson install` is run.

build-aux/meson_post_install.py

@@ -0,0 +1,12 @@
+#!/usr/bin/env python3
+
+from os import environ, path
+from subprocess import call
+
+if not environ.get('DESTDIR', ''):
+    PREFIX = environ.get('MESON_INSTALL_PREFIX', '/usr/local')
+    DATA_DIR = path.join(PREFIX, 'share')
+    print('Updating icon cache...')
+    call(['gtk-update-icon-cache', '-qtf', path.join(DATA_DIR, 'icons/hicolor')])
+    print("Updating desktop database...")
+    call(["update-desktop-database", path.join(DATA_DIR, 'applications')])