unified-selection: Unable to deselect a model by clicking in [email protected] graphics view

[grigasp]

1. This is only reproducible with [email protected].

2. Launch test app

3. Open Models tree

4. Select a Model node -> all elements in the graphics view are highlighted

5. Ctrl + Select an Element node - nothing changes in the graphics view

6. Click in an empty space in graphics view

   Expected: Selection is cleared - nothing is highlighted in the graphics view and nothing is selected in the tree.

   Actual: Tree selection was cleared, but graphics view keeps everything highlighted.

Note: likely broken with #800