nom::Text API cleanup

* math_helpers: Introduce generic set_alignment method.

* Text: Redeclare ::width and ::height methods private && remove
::set_size; the text's width and height are now accessed through the
inherited size method. Setting the size is now accessed through the
inherited set_size method.

* Text: Remove alignment functionality from class; this is now
available from nom::set_alignment.

* Text: Remove Text(const std::string& text) ctor.
* Text: Remove ::local_bounds && ::global_bounds.
* Text: Remove ::features, ::set_features && ExtraRenderingFeatures
enum.