Separate CustomEntity global positions internally

This makes it so that `CustomEntity`s, at least internally, do not use
global tile position. Instead, they will use room-x and room-y
coordinates, which will be separate from their x- and y- positions.

This makes it much easier to deal with `CustomEntity`s, because you
don't have to divide and modulo everywhere to use them.

Since editorclass::add_entity and editorclass::get_entity_at expect
global tile position in their arguments, I've added room-x and room-y
arguments to these functions too.

Of course, due to compatibility reasons, the XML files will still have
to use global tile position. For the same reason, warp token
destinations are still using global tile position too.

desktop_version/src/CustomLevels.h

@@ -10,6 +10,7 @@
 class CustomEntity
 {
 public:
+    int rx, ry;
     int x, y, t;
     //parameters
     int p1, p2, p3, p4, p5, p6;