magnifier: add magnifier input handler.

- Added ability to use mouse wheel to zoom in/out in combination with
  current 'special key to move windows' (alt by default)

- Reworked how the magnifier is enabled and disabled.  Before, you
  could enable the magnifier, and even at a magnification of 1,
  the magnifier overlay was active and the performance hit was severe.
  Now the magnifier enabled key is more of a preference.  The actual
  magnifier 'engine' is only active now when the magnification factor
  requires it.

- Moved zoom in/out via media keys to be handled in the MagnifierInputHandler.
  The media keys were simply updating the magnification in gsettings, which
  was being monitored by the Cinnamon magnifier.  Now the magnification
  is set directly in response to user input.