Panzoom improvements on startup and on side panel click

[nwittwer]

• Automatically scale & pan on startup based on content size
• When you click on an artboard in the side panel, the canvas should move to show that artboard in the center of the viewport

Alternative approach w/ transform-origin: 0 0: #239

[nwittwer]

NOTE: The clicking on an artboard in the side panel -> show that artboard zoomed in at the center of the screen is not fully working correctly. It does not always move the element to exact center, especially as more artboards are added.

Also, it does not zoom in to fit the artboard.