IDMLPackage.insert_idml(): check the Spreads content in regression tests.
Fix regression tests.

Add a default image to article-1-photo-*.idml files.
Imported XML files have a link to an image as a relative path that actually exists.
Fix ramifications due to those changes.
Add regression test for insert_idml() with a source file more complex.
IDMLPackage.insert_idml(): Only append the source spread elements that are needed into the destination package.
IDMLPackage.get_spread_object_by_xpath(): remove duplicate code.
Add regression test for insert_idml() with a src without picture.