table width specification possible, relative and % column width

- the textStyleSheet_c::getValue gets a default value as argument that
  is returned when no value is found instead of the CSS default
- templatize the evalSize function so that we can use lambdas
- add tests (of course)
replace data vector in textLayout_c with private getter function
rename textLayout_c -> TextLayout_c
rename commandData to CommandData_c

- make a proper class out of the structure
- provide constructors for the 3 available commands
- use those constructors all over the place
- still public members though... but doesn't matter here, because once
  in TextLayout_c they can't be changed
- also fix loading and saving layouts to include image width and height
  and fix all tests accordingly so that they also compare those fields
fix link movement in vertically centred boxes

- also add some comments here and there