The "Configure Toolbars" dialog was made non-modal a long time ago 
in order to allow drag-and-drop of toolbar actions while the dialog is
open. This was a bad idea because:
- It is possible to open the Configure Toolbars dialog and then any other modal dialog (e.g. Configure Shortcuts), making the toolbar dialog unusable.
- After a drag-n-drop of actions in the toolbar, the order of actions in the "Current actions" column of the toolbar dialog is not updated.
Thanks to D8773 we can show the dialog as modal and fix both issues.
If some application really wants the dialog as non-modal, it can do so
by reimplementing the KXmlGuiWindow::configureToolbars() slot.
Depends on D8773