This removes the last dependency from plasma-framework on kde4support.
This change is a bit more involved than other kde4support removals because
QSharedPointer does not provide a count() method. I therefore reworked the code
to store the SharedSvgRenderer as a QWeakPointer in s_renderers, making it
possible to detect when the last one goes away by creating a QWeakPointer guard
in SvgPrivate::eraseRenderer().
REVIEW: 114912
All cpp code moves into the src/ subdirectory, as the Frameworks policy
suggests.
Directory structure should now be in line with other, future frameworks.