Add layout guides

[joeha480]

Add optional layout guides to the specification. Specifying page regions makes it possible to improve preview accuracy (reverse translation), for example for table cells or if numbers continue over a page change.

For example:

<page>
<layout-guide>
<region rows="1"/>
<region rows="3-10"/>
<region rows="3-10" cols="-15" id="region1"/>
<region rows="3-10" cols="16-" id="region1"/>
</layout-guide>
<row>...

Regions are in document order, meaning that the text flows from rows 3-10, cols 1-15 into rows 3-10, cols 16-30. This also means that e.g. table cells can flow from one page to another, by sharing the same region id. In the above example, the first region is a header line with page-numbers etc. It could further be specified using sub-regions for each field (or not specifying the entire line in the first place), e.g.:

<region rows="1" cols="-15">
<region rows="1" cols="16-">

[joeha480]

What do you think, @BWestling?