This adds a very basic reimplementation of the annotation widget for use
in the CR window. It's not pixel-perfect, but it's close enough that
most people probably won't notice. We'll want to replace this with a
real version that's shared between the PDF reader and the rest of the
codebase.
Image annotations currently show an "[image not shown]" placeholder.
Showing images is tricky: we don't have the remote cache image, so if
the remote position has changed, we could only show an image by
rendering it from the file, and only if the file itself hasn't changed.
Just for a better user experience, we could use the local image as long
as the position and file are both the same, but that would take some
rejiggering of the CR window.
Tags aren't shown because they're not shown for CR at all, though that
could be changed in the future.
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