Fix image paste support for Safari.

Safari doesn't support HTML5 image pasting from the clipboard as per http://codebits.glennjones.net/copypaste/pasteimagedata.htm — attempting to paste currently throws an JavaScript error:

TypeError: 'undefined' is not an object (evaluating 'screenshotPasteHandler.dialogView.pasteCatcher.childNodes[0]')

This suggests the code will need to be different Safari than other browsers.

Testing notes

• Ensure latest Safari on OSX works.
• Ensure QUnit tests pass.