Avoid access to ImageDB::instance() from within Database constructor.

This commit does the following things (not ideal, but I currently don't
intend working on it any further):

1. delay merging the duplicate image infos until all images have been

read (i.e. prevent inconsistencies when merging stacks)

2. move ImageInfo::merge() into XMLDB (preventing the need to access the

ImageDB singleton before it's initialized)

3. Avoid any direct or indirect access to ImageInfo::instance in the

whole of XMLDB::Database (especially in the stack manipulation methods),
so that it is safe to call them during construction.

Probably I still overlooked something because with my test data the
stack merging spiraly into an out-of-memory situation.