Add pressed()/released()/clicked() signals to Flickable

Flickable consumes press events by necessity in order to determine whether or not the user is flicking it. The basic example in qtvirtualkeyboard.git works around this by adding a MouseArea as a child of the Flickable to intercept click events:

https://code.qt.io/cgit/qt/qtvirtualkeyboard.git/tree/examples/virtualkeyboard/basic/Basic.qml#n51

With a clicked() signal, we could simply do:

    Flickable {
        // ...
        onClicked: relieveInputFieldsOfTheirFocus()
    }

Apparently there is an issue that Flickable considers itself pressed when using a touchpad on Mac, so that these new signals would be triggered when flicking in that case, which we don't want. We'd need to ensure that we distinguish between actual clicks and flicking in this case.