Feature: `Reference Tool` now Allows Loading Images Directly from Clipboard.

This PR adds the ability to paste a reference image from your clipboard. Simply copy an image into your clipboard, open your reference tool, and click the clip icon to paste the image onto you canvas.

More work can / should be done here. It would be nice if URIs to image files (both local and http) could be loaded as if it were another file. This would require a unique 'try loading image' loop that can gracefully fail and inform the user without locking up the program.

Test Plan

Try this feature on Windows and Mac OSX. This only works when the image itself is copied into the clipboard, not the url or reference to the image.