QGraphicsAnchorLayout: Make fair distribution better when we have center anchors.

 // itemX has a minimum size of 50
 // itemA and itemB have minimum size of 15

 // a sequence A and B
 addAnchor(layout, Qt::AnchorLeft, itemA, Qt::AnchorLeft);
 addAnchor(itemA, Qt::AnchorRight, itemB, Qt::AnchorLeft);
 addAnchor(itemB, Qt::AnchorRight, layout, Qt::AnchorRight);

 addAnchor(itemX, Qt::AnchorLeft, itemA, Qt::AnchorHorizontalCenter);
 addAnchor(itemX, Qt::AnchorRight, itemB, Qt::AnchorHorizontalCenter);

|                                                                                    |
|     +------------+                                              +------------+     |
|     | 15 30 60   |                                              | 15 30 60   |     |
+-----+            +----------------------------------------------+            +-----+
|     | itemA      |                                              | itemB      |     |
|     +-----+------+                                              +------+-----+     |
|           |                       +------------+                       |           |
|           |                       | 50 70 90   |                       |           |
|           +-----------------------+            +-----------------------+           |
|                                   | itemX      |                                   |
|                                   +------------+                                   |
|                                                                                    |