Drag/drop improvements for importing images

- Dragging an image over an existing image will now trigger "add as
layer"
- Dragging an image over the main form's chrome or the image tabstrip
will trigger "add as new image"
- URLs are now supported as sources

Firefox works wonderfully with this approach, as it will supply any
dragged images as 32bpp BMP files.  Chrome and IE are more finicky; if
an image links to a URL, they will provide the target URL as a file
name.  So I'll need to add extra checks for vbCFBitmap data (which they
presumably provide).
Clean up and condense drag/drop code into single function

Also, support for raw bitmap data has been added, along with a few other
miscellaneous bugfixes.
Add OLE drop support to Layers box

Now you can drop images directly onto the layers box (to add them to the
current image as new layers).