Use action function return value to make state jumps happen

- The A_Jump family of action functions now return the state to jump
  to (NULL if no jump is to be taken) instead of jumping directly.
  It is the caller's responsibility to handle the jump. This will
  make it possible to use their results in if statements and
  do something other than jump.
- DECORATE return statements can now return the result of a function
  (but not any random expression--it must be a function call). To
  make a jump happen from inside a multi-action block, you must
  return the value of an A_Jump function. e.g.:
    { return A_Jump(128, "SomeState"); }
- The VMFunction class now contains its prototype instead of storing
  it at a higher level in PFunction. This is so that
  FState::CallAction can easily tell if a function returns a state.
- Removed the FxTailable class because with explicit return
  statements, it's not useful anymore.

src/dobjgc.cpp

@@ -73,7 +73,7 @@
 #include "po_man.h"
 #include "autosegs.h"
 #include "v_video.h"
-#include "textures.h"
+#include "textures/textures.h"
 #include "r_utility.h"
 #include "menu/menu.h"
 #include "intermission/intermission.h"