ScaleManager: additional cleanup / fixes

- Sizing events:
  - CHANGE: The `onResize` callback is called only from `preUpdate` (which
    may be triggered from a window resize) and it will be called on
    refreshes even if the parent size has not actually changed.
  - A new `onSizeChange` Signal has been added. It is called _only_ when
    the Game size or Game canvas size has changed and is generally more
    applicable for performing layout updates.

- Game documentation now links to ScaleManager#setGameSize (which was
  renamed from #setGameDimensions)

- Removed extra/legacy full-screen restore code

- Margins:
  - Added `noMargins` flag; if set to true the Canvas margins will never
    be altered. This also means that
  - Margins are now set/cleared individually to avoid conflict with
    'margins' style compound property

- Code consistency updates
ScaleManager: back-port of "window constraints".

- Adds `ScaleManager#windowContraints`
  - In 2.1.3 and prior the scale modes (EXACT_FIT, SHOW_ALL) were actually
    based off the window dimensions, even though the parent element did
    not correctly reflect this nature.
  - When set (the default now is that right and bottom are set) the
    behavior will mostly correctly mimic the 2.1.3 (minus bugs) and
    before.
  - CHANGE (from 2.1.3): The window constraints also affect the RESIZE
    mode, arguably this is more consistent.
  - To disable this "constrain to window" behavior, simply set the
    appropriate property to false, as in:
    `game.scale.windowConstraints.bottom = false`