Make _xPos and _yPos consistent across every Class

[ArifRoktim]

The meaning of _xPos and _yPos is different across the classes.
It is either:

1. The column/row number of that object with respect to the 2d array of Tiles in the Map class
2. The actual x and y pixel coordinates of the object, which is the above number times 40.

I think we should make all occurrences of _xPos and _yPos consistent across every class (specifically, to mean the first of the above points) because otherwise comparing coordinates would involve having to remember if _xPos means the position in the array or the pixel coordinate.