Add set-target SEXP operator

[naomimyselfandi]

Targeting is one of FSO's most fundamental mechanics, so it makes sense to let mission events mess with it. Create an operator which can set the player's target ship and, optionally, target subsystem; provide some way to clear the player's target as well.

(This can already be done through scripting, but a builtin operator makes it more accessible.)

TBD: If a subsystem is specified, but destroyed, does the ship still get targeted? (Leaning towards yes, but I could go either way as long as it's documented.)