ButtonGroup needs a currentIndex property

TabBar's currentIndex property is convenient for bindings and such. But a mutually-exclusive set of buttons is almost the same thing, and it should be similarly easy to use.

It came up in https://codereview.qt-project.org/c/qt/qtdeclarative/+/368209/2 : we need buttons to choose the "tool" that controls what the mouse will do when you click or drag on empty space on the drawing canvas. An integer can be used to look up something in an array of tool-related things, for example (although that's not the only way to do it), in a switch statement, etc.