The basic design is:

1) Each element of the text (text, tspan) is represented with
   KoSvgTextChunkShape. This chunk stores all the information
   about this portion of the text. The properties are stored in
   KoSvgTextProperties object, that will allow us to export them
   as a markup.

2) A subtree of test elements is stored in KoSvgTextShape, which
   is also a chunk.

3) The main text shape accesses the internals of its chunks with
   KoSvgTextChunkShapeLayoutInterface. It encapsulates all the
   details that should not be accessed in public.

4) A text chunk can be "a text" or "a node". In the latter case
   it is just an intermediate node that stores a set of children
   that actually have the text.

5) The layout of the text is performed by KoSvgTextShape