WIP: Add image download button

[almccon]

Description

For most workflows the "Copy image" button is all we need, but sometimes (especially for my half-baked, half-automated QA process that I'm working on for one of our clients) I want to download the screenshot instead of save it to the clipboard. I can do that now, but it takes a few extra steps (open Preview, ⌘N to create new image from clipboard, then save the file, manually copying useful information into the filename).

This PR adds a new button (unlabeled to safe space, with just a download icon) next to the screenshot icon, that saves the current map view to a file and adds the zoom and lat/lng to the filename. It somewhat streamlines my workflow.

I renamed the existing downloadScreenshot function to copyScreenshotToClipboard, and my new function uses the old downloadScreenshot name. But currently they're duplicating most of the code, which isn't great.

In the longer term I'd like to think about how to hook this up to a list of QA locations in the gazetteer and download several of them all at once, and probably also have the gazetteer define a prefix to use for the downloaded images.

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