libdeng2: Use the convenience typedef TimeDelta

Instead of spelling out Time::Delta.
Shell|libdeng2: Working on rules

Todo: Rule dependencies and ownership need to be reference-counted.
libdeng2: Added de::refless()

de::refless() is a function to be used when allocating new reference
counted objects. It removes the default +1 reference count of the
new object, with the assumption that a reference is taken by someone
else soon afterwards.
libdeng2|Refactor: Rules are reference-counted

Simplified the lifetime and ownership management of rules by applying
reference counting.

Also added functions de::holdRef() and de::releaseRef() for more
convenient/safer reference holding and releasing.
libdeng2: Use holdRef() and releaseRef()