ScaleManager: additional cleanup

- Added `compatibility` settings
  - CHANGE (2.1.2-4): moved `supportsFullScreen` and `noMargins` into it
  - Added additional properties for greater control and up-front settings.
    - `scrollTo`: where the browser will scrollTo, if anywhere
    - `forceMinimumDocumentHeight`: apply document element style?
    - `allowShowAllExpand`: allow SHOW_ALL to try to expand? (It already
      could, this allows configuration.)

- Removed `windowConstraints.top/left`. This may be a feature in the
  future, but scrubbed for now.

- Added `USER_SCALE` scale mode. This is like NO_SCALE but it scales off
  of a user-specified scale factor, as set by `setUserScale`. This is
  marked as "experimental" as the exactly semantics of non-adjusting modes
  (e.g. NO_SCALE and USER_SCALE) wrt. Canvas and "maximum" size clamps
  need to be re-examined.

- FIX: `onSizeChange` now works as documented, which means it is also
  fired if the game size changes even though the game canvas size does
  not.

- CHANGE (no known breaking): `margins` is now non-Point/non-Rectangle
  that uses top/left/bottom/right properties (any quasi-updated x/y). This
  is to get around the issue that Rectangle is only designed for positive
  width/height cases.

- Cleaned up property access / quotes for consistency

- Various documentation cleanup and consistency

- Fixed issue with not clearing an unparented `_createdFullScreenTarget`

- Added Phaser.Rectangle.sameDimensions which does a strict equality check
  over the `width` and `height` properties of two objects, perhaps
  Rectangles.

Author: pnstickne