common: Add Deferred{Unique|Shared}Ptr and helpers

This commit adds smart pointers for holding QObject derivatives.
On destruction, they will deleteLater() the object they're holding.
This is the recommended and safe way for deleting QObject instances.
Be careful with using such smart pointers outside of the scope of
their parent, for objects that are owned. The parent will delete
its children, and any outside reference still held by a smart pointer
will lead to a double free!