Feature: Replace text in text-boxes

[t-oster]

Hi,

this is my attempt at thombergs/docx-stamper#97. The approaches from thombergs/docx-stamper#97 did not quite work, because they broke some tests.

I added it in a more conservative way: I search through all P elements in the document and find those, which are children of Text-Boxes. This is added as an additional helper method ParagraphUtil.getAllTextBoxes(document).

In the CoordinatesWalker.walk() method I run walkContent on those elements after the other content is processed. This way I did not break any tests. I added a test document with two simple text-boxes and it works.

Things I noticed:

• My ParagraphUtil.getAllTextBoxes(document) finds all text-box paragraphs (in the test-document) twice with different parents. However this doesn't seem to be a problem (I guess the text is processed the first time and the second time it already contains the processed text)
• I did not try this with recursion or start/end comments.

However since the current implementation just ignores text-boxes this is IMHO safe to add, because it just adds functionality, which may not work in complex cases.