Behavioral incompatibility between QQuick* checkables and QWidget/Quick.Controls checkables

It is impossible to use the QQuickAbstractButton's `clicked` signal and `checked` property in a similar way we did for years with QAbstractButton and/or Quick.Controls Button (same for other checkables).

some parts of a whole issue:

• when the slot connected to the CheckBox' clicked() signal explicitly sets its checked state (i.e. in tristate case), the checkedChanged() signal could be emitted more than once per click, due to nextCheckState() call
• in some cases Switch doesn't emit `clicked` and thus it is not always possible to distinguish "toggle due to the user action" and "toggle due to the 'checked' property binding re-evaluation"; it seems like `clicked` means the mouse click here, not the button click.

(see QAbstractButton::clicked() documentation for more differences)

The following code could be used to see this issue in action:

QtObject {
    id: action
    property bool checked: false
    function toggle() { checked = !checked; }
}
Button {
    text: "First"
    highlighted: checked
    checked: action.checked
    onClicked: action.toggle()
}
CheckBox {
    id: button
    text: "Second"
    checked: action.checked
    onClicked: action.toggle()
}
Switch {
    text: "Third"
    checked: action.checked
    onClicked: action.toggle()
}