[gtkmm] Gtk::Application::custom_class_init(): Update a comment

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 0b8cd52f2c493dcda98258d970778f2d55ea21c5
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Sep 27 16:17:21 2017 +0200

    Gtk::Application::custom_class_init(): Update a comment
    
    The private custom_class_init() method is still needed, but the
    reason is no longer that the deprecated g_type_init() must be called.

 gtk/src/application.ccg |    6 ++++--
 gtk/src/application.hg  |    6 ++----
 2 files changed, 6 insertions(+), 6 deletions(-)
---
diff --git a/gtk/src/application.ccg b/gtk/src/application.ccg
index d2ef1fe..8aa5ad6 100644
--- a/gtk/src/application.ccg
+++ b/gtk/src/application.ccg
@@ -91,9 +91,11 @@ const Glib::Class& Application::custom_class_init()
 Application::Application(const Glib::ustring& application_id, Gio::Application::Flags flags)
 :
   // Mark this class as non-derived to allow C++ vfuncs to be skipped.
-  //Note that GApplication complains about "" but allows nullptr, so we avoid passing "".
+  // GApplication complains about "" but allows nullptr, so we avoid passing "".
   Glib::ObjectBase(nullptr),
-  Gio::Application(Glib::ConstructParams(custom_class_init(), "application_id", (application_id.empty() ? 
nullptr : application_id.c_str()), "flags", GApplicationFlags(flags), nullptr))
+  Gio::Application(Glib::ConstructParams(custom_class_init(),
+    "application_id", Glib::c_str_or_nullptr(application_id),
+    "flags", static_cast<GApplicationFlags>(flags), nullptr))
 {
   // gtk_init() is called by the 'startup' default signal handler when g_application_run() is called.
   // It's also called here, to make it possible for users of gtkmm to create
diff --git a/gtk/src/application.hg b/gtk/src/application.hg
index 8d2d8c2..b275051 100644
--- a/gtk/src/application.hg
+++ b/gtk/src/application.hg
@@ -307,10 +307,8 @@ public:
   _WRAP_SIGNAL(void window_removed(Window* window), "window-removed")
 
 private:
-  /** This is just a way to call Glib::init() (which calls g_type_init()) before
-   * calling application_class_.init(), so that
-   * gtk_application_get_type() will always succeed.
-   * See https://bugzilla.gnome.org/show_bug.cgi?id=639925
+  /** This is just a way to call Glib::init() before calling a Glib::Object ctor,
+   * so that glibmm's GQuarks are created before they are used.
    */
   const Glib::Class& custom_class_init();