Drop support for gtk+ < 2.24
Remove deprecated call to g_type_init
Cleanup GTK_CHECK_VERSION and unify gtk+2/3 code where possible

Always use gdk_window_get_display and GDK_WINDOW_XID
Replace GtkObject with GObject
Always use gtk_message_dialog_get_message_area (since gtk 2.22)
Unify notebook code between gtk2 and gtk3
Use entry-text-column instead of text-column
Remove has_separator property from gtkbuilder files
Bump required version for gtk+ ang glib
Remove duplicate GTK_REQUIRED