Add canvas-level annotation target

[cbeer]

Split from #4

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This is different from the other target drawing tools, so is in a separate section on the Target toolbar.

• The icon is post_add: https://material.io/resources/icons/?search=post&icon=post_add&style=sharp

• Tooltip is Entire page:

• Behavior: Ideally, when the user selects the Entire page icon, we immediately display a target shape (which is not resizable by the user) that encompasses the current canvas:

Note that I closed the sidebar in the mockup above just to illustrate the canvas selection more clearly; I'm not suggesting we would close the sidebar when the user selects the Entire canvas target action.