QQC2 Button lacks isDefault property from QQC1

Nuff said, there is no more isDefault property in QtQuick.Controls 2 framework.

I discovered this property while cleaning up org.kde.desktop QQC2 style implementation, and found this leftover:

     background: StylePrivate.StyleItem {
        // ...
        activeControl: controlRoot.isDefault ? "default" : ""

The code still "works" without warnings because controlRoot.isDefault simply evaluates to undefined, and so the whole expression is always just an empty string.

I didn't find any suitable replacement in the new API, so I consider this a regression.