Standardise transform aliases on GameObjects and Groups

Re Github issue #67

Add anchorPointX, anchorPointY aliases to Transform. These point to rotPointX and rotPointY respectively.

Comply the IChild family of game objects (including Group, State, Entity, Sprite, StaticImage, TextField, and TileMapLayer) to this list of aliases:
x
y
worldX (get only)
worldY (get only)
rotation
rotPointX
rotPointY
anchorPointX
anchorPointY
scaleX
scaleY
scale (set only)

Add scaleToWidth() and scaleToHeight() methods to Entity. These set the scale relative to the width and height properties.

Disable scaleToWidth(), scaleToHeight(), and cellIndex getter/setters on TileMapLayer, which has width and height properties that mean something different to those inherited from Entity.

Add properly calculated width and height to TextField.