Define XDDF user model for shape properties to be shared between XSSF and XSLF
This pull request is branched from the `xddf-usermodel` branch (pull request #68).
The main goal of this work is to provide a user model API in order to handle the shape properties without using underlying xmlbeans objects and interfaces. Among others, handling properties related to colors is made simpler by returning and accepting an abstract XDDFColor, which has six distinct implementations, where the developers were required to check whether or not one of the six properties was set.
What is left to be done:
- refactor XSLFColor and XSSFColor to make use of the new shareable code;
- deprecate methods that return xmlbeans objects where a user model object now exists.
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