DOM Functions - their own space

No known breaking changes - as it's still dev/internal stuff.

- Added Phaser.DOM to house new DOM functions, moved stuff
  over from ScaleManager as appropriate

- Fixed a fiew cases of missing functions

- Changed some of the new signatures to protected for the interim.
  (Maybe a `beta` tag would fit better? Public is promises!)

- Moved generic support from Canvas to DOM and added proxy/notes
  - Updated internal usages

- Updated some comments for consistency
  - Access always on bottom for members/properties, public assumed