Provide easy LTR / RTL switch for user

While refactoring the css for the inner content (#35 (closed)), I noticed the entanglement of the current language direction with the indentation plus some side-boxes (notes). With my fixes for #35 (closed), the user is already to easily customize the text direction and the margin. But it's not possible to easily customize notes in the sidebar.

In order to provide the requested feature "User can switch text direction for the inner content in one central file/interface", a mixin has to be added, so margins will adapt automatically according to the direction.