Add .desktop file

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

data/meson.build

@@ -0,0 +1,16 @@
+desktop_file = '@0@.desktop'.format(base_id)
+
+if desktop_file_validate.found()
+  test(
+    'validate-desktop',
+    desktop_file_validate,
+    args: [
+      desktop_file
+    ],
+  )
+endif
+
+install_data(
+    desktop_file,
+    install_dir: datadir / 'applications'
+)