Shared Preview Selection Model

[RobertBColton]

Following up on #179, right now the path preview does not use an actual QItemSelectionModel and does not support multiple point selection. It would be possible for the preview to share the same selection model as the points table list. If it did use the model a lot of the selection logic in the path editor could be simplified.
https://doc.qt.io/qt-5/qtwidgets-itemviews-chart-example.html
It would be useful for editing to select several points and move them together. This behavior would also be desired in the room editor where we can consider doing the same thing. Finally, we could also correctly use the current style to paint selection and focus according to the platform conventions, like QGraphicsView/QGraphicsScene does with selected items.