pptx: Positioning of paragraphs without label definition.

* Let's treat all not empty paragraphs as list-items. The positioning of paragraphs in a text-box by MS PowerPoint
differs from positioning of paragraphs inside text by MS Word. This simple modification does not require any change
in the layout. However we still have to support LO/OOo, who do not use this approach. Kind of strange because
their ppt filters do. The problem is that a paragraph must be positioned inside of the text-box. So the indent > margin
(hanging mode) case has to be handled.

* The logic to position list-items wo a label differs compared to labeled list-items in MS PowerPoint.
I updated the logic and tried to make the logic for labeled list-items more readable.