Fixed: Warnings given by new version of clang

Some possible use of uninitialized variables.
Fixed|libshell|libdeng2: DENG2_PIMPL required by DENG2_PRIVATE

The private instance must be derived from de::IPrivate for destruction
to work correctly.
Merge branch 'master' of ssh://github.com/skyjake/Doomsday-Engine
Fixed: Minor warnings (initialization order)