Image scaledWidth/Height returns a Number now

Image scaledWidth/Height, as well as Text scaledWidth/Height, will
return a Number one, instead of an uint.
Make scaledWidth and scaledHeight settable properties
Switching world A->B->A in same frame goes to world B
Improve documentation for Image properties.
Document frameRate argument for Spritemap.add()
Improve documentation of Text.setStyle() and Text.richText.