Update model view to make tables fit on the canvas better, updating 'fit to screen'

Model view places new tables to the top right of the existing canvas. This can lead to incredible wide canvases, with tables that need to be dragged back to the left of the screen. New tables could instead be placed into free spaces in the canvas. Secondly, the 'fit to screen' option will only 'fit' to the top left of the page, even if no tables are shown there and the majority of them are clustered in the centre of the canvas. There is room to improve both of these features.