Add 2D board fullscreen toggle

[ecurtiss]

When I write on my tablet, I feel like I'm restricting my thoughts to a small screen. Then, when I write on a metaboard, I feel like I'm restricting my thoughts even further to a virtual board on a small screen. I want to be able to full screen the 2D board to regain some of my brain power. This would also make writing on small phone screens easier.

[blinkybool]

We'd either need to let you hide the toolbar, or let you zoom into and drag around the canvas. Zooming and dragging is difficult because the lines are positioned absolutely, not relative to a Frame, so the lag would be immense when updating every line.

Perhaps when you zoom, you just see the border, and it triggers a re-render when you let go?

Toolbar hiding does seem simpler, and when it's hidden, the canvas expands as far as it will go (or maybe it's always expanded and the toolbar goes over the canvas when shown?)