QmlDesigner: Add base line anchoring (for text items)

Baseline anchoring is an important feature for positioning TextItems with different fonts on a common base line. It is present as feature in the Text base item, but it is missing in Qt Quick Designer's Layout/Anchors properties.

Baseline alignment was a strong request at customer meetings, and it took us (who did the customer workshop) a while to find it and demonstrate it, would have been great if the Layout/Anchors gui had made it discoverable.