Add styles for nested "draw:*" elements when cloning master pages.

Previously only the top-level `draw:*` elements were styled.
I should have proposed some text to guide you, sorry. Idea of the ChangeLog.md is to tell end-users what to expect from a version (so if it is worth trying an upgrade, not us code-insiders :) So "cloned master pages" might leave most puzzled. Something like
"* More elements from master pages are now correctly positioned when displayed inside slides."
might be better