Extract existing annotations on PDF import

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Keypoints should offer to extract any(*) existing PDF annotations when a PDF gets imported into a Keypoints document.

(*): Note that this mostly makes sense for annotations that enclose some text (like a highlight annotation where some text is highlighted in the PDF). When image support has been added, rectangle annotations that enclose an image should be also supported. But it might make less sense for handwritten annotations or stamp-like annotations (like a question mark in the margin).

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I have started to build the necessary underpinnings but this is a bigger task that may take some time to get right.

I'm also planning an "Import Selected Skim annotation(s)" script which may arrive first.