Added image drag-and-drop and embedding

* Dragging and dropping an image file into the story map will embed it
- a passage, tagged 'image', containing the file encoded in base64, will
be added. Thus, images can now be saved inside the .tws file and the
compiled HTML.
* Supported image file extensions are GIF, PNG, JPEG, SVG and WebP.
* The [img] syntax has been updated to allow 'image' passages to be
referred to in addition to filenames. This allows embedded images to be
used multiple times in a story, without redundant extra copies being
included.
* Added PassageWidget colouring for 'image' passages.
* Also added an 'Import Image' menu item to the Story Menu.