Covers Heading 1 - Heading 9

Example available in examples under DocumentHeading() method.

Systax:

SetMargin(float top, float bottom, float right, float left)

by
Shashwat Tripathi

Removed unused variables from _Extensions.cs

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Created an abstract class called DocXElement and made every document element such as {Table, Row, Cell, Paragraph, Run, Text, etc} extend it.
Created an abstract class InsertBeforeOrAfter which is derived from DocXElement. This provides functions for inserting, page breaks, Paragraphs and Tables before and after self.
Moved Text and Run inside Paragraph, keep consistancy with Table which contains Row and Cell.
To Do: Investigated why the Picture class had two internal constructors. I removed one.
Added a _ to each .cs file that does not contain instanceable types, {Enumerations.cs, Extensions.cs, etc}

Bugs fixed
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Cell had a parameter Paragraph, this was incorrect as paragraphs can contain multiple Paragraphs. Now it returns a List<Paragraph>
InsertDocProperty now returns a DocProperty instead of void.

New features
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InsertDocProperty now contains overloads for the parameter track changes.
Cell now contains a Shading property, this can be used to set the background color of a cell.