Core/AI: Factory functions cleanup

- Get rid of fugly void pointers
- Streamlined AI selection code, using Permissible as it should've been instead of harcoded checks on the selector code
- Moved IdleMovementGenerator singleton to factory code

src/server/game/Movement/MovementGenerator.cpp

@@ -17,5 +17,12 @@
  */
 
 #include "MovementGenerator.h"
+#include "IdleMovementGenerator.h"
 
 MovementGenerator::~MovementGenerator() { }
+
+MovementGenerator* IdleMovementFactory::Create(Unit* /*object*/) const
+{
+    static IdleMovementGenerator instance;
+    return &instance;
+}