ODT reader simply provided an empty header list
which meant that the contents of the whole table,
even if not empty, was simply ignored.
While we still do not infer headers we at least have
to provide default properties of columns.
Frame can contain other frames with the text boxes.
This is something that has not been considered before
and meant that the whole construction of images was
broken in those cases. Also the captions were fixed/ignored.
Review revealed that we didn't handle the case
when the starting point is an empty string. While
this is not a valid .odt file, we simply added
a special case to deal with it.
Also added tests for the new feature.
