Improve display & navigation around ODF fields

Improving support within WebODF for fields. This doesn't implement any specific handling for any field. The most noticable change can be found by loading up the visual-tests document in current master, selecting the complete document and removing it's content. In current master, this leaves behind a lot of fields + text elements.

* Includes a tool to help parse the RNG to figure out text containers
* Add demonstration section to visual-tests document (http://localhost:8124/programs/editor/src-localeditor.html#../../webodf/tests/visual-tests.fodt)
* Visually distinguish fields from non-field text regions

Fixes #772 

![field-improvements](https://cloud.githubusercontent.com/assets/1052079/3961668/39df356e-2759-11e4-9d22-9518c01e8f7a.png)