Provide the page body in the skin as individual top level paragraphs instead of one big blob

Currently a skin can refer the body of the processed page only as one big blob that can be inserted somewhere. For more advanced layouts it would be useful to access the individual top level paragraphs individually in a list.

The new TopLevelBlockExtractingRule will extract all HTML block elements at top level (i.e. direct children of body) into separate elements and provide them as list of paragraphs in the skin. Although a body may contain only such block elements, some pages might add other ones or even plain text in the body. Such parts are added as own elements in a new paragraph (p) element at the proper position in the paragraph list.