So Anjal can override what it needs to for its own purpose.
Also makes the EShellWindow design a little cleaner.

Methods added:

  GtkWidget *    (*construct_menubar)   (EShellWindow *shell_window);
  GtkWidget *    (*construct_toolbar)   (EShellWindow *shell_window);
  GtkWidget *    (*construct_sidebar)   (EShellWindow *shell_window);
  GtkWidget *    (*construct_content)   (EShellWindow *shell_window);
  GtkWidget *    (*construct_taskbar)   (EShellWindow *shell_window);
  EShellView *   (*create_shell_view)   (EShellWindow *shell_window,
                                         const gchar *view_name);

Also added some new GObject properties to help decouple actions from
internal EShellWindow widgets created by these methods:

  EShellWindow:sidebar-visible
  EShellWindow:switcher-visible
  EShellWindow:taskbar-visible
  EShellWindow:toolbar-visible