fix #458 error when no skin and no widgets loaded

Due to a bug in the code delivered in 59e9ba72a65c810df7935c5a1a4195365e44d233
(pull request #427), when no skin and no widgets were loaded, JavaScript
errors were thrown due to nullpointer exception.
refactor Two new features have been added in the LayoutTester:

  - The elements positionning are now relative to the root element (default to body)
  - An error margin can be given for more stable tests

Moreover, some vertical alignement tests have been added.

Close #457
1.4.1