ScaleManger - better FS init, documentation

- Added `onFullScreenInit` signal which is the correct place for a client
  to alter the FS target element, such as to set a background color or add
  a CSS class.

- Increased documentation overview and specificity including expected
  Parent behavior, sizing calculations, and when `refresh` may be required.

- Grouped documentation for scale modes (in 'scaleMode')

- Separated out internal/deprecated `setScreenSize` method from
  `updateLayout`.

There are no known breaking changes.