Make set_image_count() efficient with large values.

[riccardomurri]

The current implementation of get_image_count() scans the XML to find all image elements; on the other hand, set_image_count() adds those elements 1 by 1, calling get_image_count() (via the self.image_count property) every time. This results in a quadratic complexity algorithm which gets really slow when the number of images number over a few 10's of thousands.

This PR fixes the issue by:

• caching the value of get_image_count() in an attribute self._image_count
• ensuring that get_image_count() is called at most 2 times from within set_image_count()

The resulting behavior is that set_image_count() runs in linear time for every value.

(Issue reported by --and patch co-developed with-- @scottberry )