Embedded image previewing added

* Base64 image passages can now be opened to display the image. The
ImageFrame window currently only shows static images, with no GIF
animation, and large images cropped at 1024x768.
* The ImageFrame window's menu includes 'Replace Image', which can
replace the image with a different file.
* The tag that denotes an image passage is now 'Twine.image' instead of
'image'. This tag can not be given to text passages, nor removed from
image passages.
* Fixed drag-and-drop not working with non-lowercase file extensions.