Refactoring to make the variable height more important

* @init_width and @init_height removed now there is only
  @fixed_height
* setup_dimensions from contents_alginment removed, calling it
  once in initialize should be enough
* Constants remvoed as zero and 1.0 are more apparent there and
  only used there which kind of makes it clearer I believe
* some tests added to make sure everything still works (e.g.
  tests that check if the content is bigger and smaller than the
  set height, the height still stays the same)