extensionPrefs: Use imports.package.start()

We want to make the extensions app code more self-contained to make it
easier to build separately, and ultimately make it available on flathub.

One complication we are facing is that it is currently all over the source
tree:
 - js/extensionPrefs for the main code
 - src for the launcher process
 - data for .desktop file and icons

Switching from a C launcher to the imports.package module allows us to
consolidate the first two, and will also take care of the annoying
setup bits (defining JS search path, extending GI lookup, loading
resources).

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1081

src/gnome-shell-extension-prefs.c

@@ -1,51 +0,0 @@
-/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
-
-#include "config.h"
-
-#include <gjs/gjs.h>
-#include <glib/gi18n.h>
-
-int
-main (int argc, char *argv[])
-{
-  const char *search_path[] = { "resource:///org/gnome/shell", NULL };
-  GError *error = NULL;
-  GjsContext *context;
-  int status;
-
-  bindtextdomain (GETTEXT_PACKAGE, LOCALEDIR);
-  bind_textdomain_codeset (GETTEXT_PACKAGE, "UTF-8");
-  textdomain (GETTEXT_PACKAGE);
-
-  context = g_object_new (GJS_TYPE_CONTEXT,
-                          "search-path", search_path,
-                          NULL);
-
-  if (!gjs_context_define_string_array(context, "ARGV",
-                                       argc - 1, (const char**)argv + 1,
-                                       &error))
-    {
-      g_message("Failed to defined ARGV: %s", error->message);
-      g_error_free (error);
-      g_object_unref (context);
-
-      return 1;
-    }
-
-  if (!gjs_context_eval (context,
-                         "const Main = imports.extensionPrefs.main; Main.main(ARGV);",
-                         -1,
-                         "<main>",
-                         &status,
-                         &error))
-    {
-      g_message ("Execution of main.js threw exception: %s", error->message);
-      g_error_free (error);
-      g_object_unref (context);
-
-      return status;
-    }
-
-  g_object_unref (context);
-  return 0;
-}

src/meson.build

@@ -249,15 +249,6 @@ executable('gnome-shell', 'main.c',
   install: true
 )
 
-executable('gnome-shell-extension-prefs',
-  'gnome-shell-extension-prefs.c', prefs_resources,
-  c_args: tools_cflags,
-  dependencies: tools_deps,
-  include_directories: [conf_inc],
-  install: true
-)
-
-
 if have_networkmanager
   executable('gnome-shell-portal-helper',
     'gnome-shell-portal-helper.c', portal_resources,