Let Ctrl+0 zoom and center the viewport on the artwork

[Keavon]

Ctrl0 should pan and zoom the viewport to fully display the content. If the canvas is rotated, it should still achieve this goal of showing all the content (so, for example, if the content was just a rectangle, and the canvas was rotated, the top/bottom or left/right should end up touching the edges of the viewport). When we have implemented artboards (#289), this will instead frame the artboard rather than the bounding box of the content.

These bounds also form the basis for the scrollbars and rulers in the document, and also the dimensions for an exported SVG file (which is currently harded at 1920x1080 centered around the document origin).