Add entityclass::removeentity()

Ok, once we switch everything over to not use the 'active' system, it's
easier to read removeentity(t) than it is to read
entities.erase(entities.begin() + t).

desktop_version/src/Entity.h

@@ -64,6 +64,8 @@ public:
 
     void createblock(int t, int xp, int yp, int w, int h, int trig = 0);
 
+    void removeentity(int t);
+
     void removeallblocks();
 
     void removeblock(int t);