Add the ability to set the defaultAction on a QToolButton in Designer

Currently, there does not appear to be a way to associate an action with a standalone QToolButton from within Qt Designer. This means that A) dialogs that use QToolButtons for buttons with icons don't get a proper preview in Designer, instead having to use things like placeholder text, and B) that the programmer must manually call QToolButton::setDefaultAction in the widget/dialog constructor in order to get clicks and such working properly.

It would therefore be useful if Designer provided the ability to set the defaultAction on a QToolButton from among the actions defined for the dialog.