Object Positions

[Zulu-Inuoe]

Creating this from #14

Tiled Objects (in an Object Layer) have 'surprising' behaviour with regards to the meaning of the X and Y coordinates:

1. Rectangles and ellipses without width/height values use X/Y as their center
2. Rectangles and ellipses with their width/height values set use X/Y as their top-left corner
3. Polygon objects use X/Y to position the first point in the polygon. Rotations rotate about this point
4. Tile Objects use X/Y as the bottom-left corner
5. Text Objects use X/Y as their top-left corner

These rules apply even for collision objects in the Tile Collision Editor.

We'd like to come up with a way for users of this library to not have to implement this logic over and over.
My proposed fix would be to keep this code as-is and consider it the 'low level interface'. Then provide a 'high level interface' that provides better-behaving position functions to make it easier for library users so they don't each have to encode these very specific rules in order to get a 1-to-1 with the Tiled editor.

More concretely, something like providing places for object-center, object-left, etc.
We can also toss other utility style functions into this 'high level interface' to overall ease making use of Tiled.

Thanks!