Fix loading loading SVG's <use> tags, which source is after the use point

This patch implements a special DeferredUseStore object that keeps the
list of unresolved <use> tags and resolves them when they become ready.

The original patch was written by
Mikael Rosbacke <mikael.rosbacke@gmail.com>

I just renamed a few symbols and fixed indentation to fit KDE's
coding policy.