Add command to screenshot the whole viewer

[emilk]

For our docs we want to produce nice screenshots like these:

This was made using cmd-shift-4 on a Mac. Note the transparent background with the shadow.

It would be great to have a simple, unified method to produce these screenshots across different OS:es, available via a the command palette and a keyboard shortcut.

We could, for instance, just read back the framebuffer, round the edges, add some spacing and shadow, and then save that as a png. We wouldn't get the mac-style close buttons though (the red,yellow,green in the top left), so we would need something like #1063 for that, or just fake it

[emilk]

Niko just said he wants it too look like the web instead of native which means:

• No rounded corners (we can do it in CSS instead)
• No top-left native close/maximize/minimize-buttons
• Style like a web page (thick top bar)
• No drop-shadow (we'll do it in CSS instead)

This would make it easier to implement.