Don't load embedded images from the current working directory.

Override QTextImageHandler's image loading as it will attempt to resolve
relative paths and load the image itself if the document returns an
invalid image from loadResource, which we don't want as it bypasses the
pixmap cache and resolves against the application and current working
directory instead of the Text items context.

Change-Id: Ia1d3633036f96d902e1ac03dae5d5b203fba7ff1
Reviewed-by: Martin Jones <martin.jones@nokia.com>