GCC has some optimization for the inclusion guard, but they only work if
the check is the outermost one.
We're fairly inconsistent because of historical reasons, so we should
ensure that we follow the same pattern in every public header.
Step two: rename the object and its methods.
While we're at it, adhere more strictly to the coding style
practises; rename :clone-source to :source; add a setter method
for the :source property; take a reference on the source actor
to avoid it disappearing while we're still accessing it.
