[toolbar] Locked toolbar pushes page up (about 1gu)

[Tim Peeters]

When a toolbar is locked, it pushes the Page up about 1 gu that is kept blank between the toolbar and the Page.
This can be shown by adding:

    Rectangle {
        color: "orange"
        anchors.fill: parent
    }
to the Page inside tests/resources/navigation/MyCustomPage.qml in the UITK code, and then opening that page via PageStack.qml.

(my screenshot functionality is currently broken, I'll try to add a screenshot later).

I verified the bug on phone (enter selection mode from gallery-app events view) and desktop (PageStack example with the modification listed above).