Fixed toolbar icon size.
Use GTK_ICON_SIZE_LARGE_TOOLBAR and GTK_ICON_SIZE_SMALL_TOOLBAR instead of
GTK_ICON_SIZE_MENU and GTK_ICON_SIZE_BUTTON
Fixed Bug 727914 "Logo is not shown in About-dialog"

Made GtkAboutDialog logo and logo-icon-name properties optional since they
are mutually exclusive.
The editor is responsible for enabling then accordingly.