oll-doc: rename field names

It seems necessary to have *all* fields prefixed with "snippet-".
This is to avoid ambiguities.

Additionally this commit stores the determined \version string
as a field "snippet-version" in the headerFields dictionary.
oll-doc: Tiny line breaking improvement
oll-doc: Make HTML generation object oriented

The initial approach was extremely hacky, now we have proper HTML
objects making use of the template pattern and inheritance.
oll-doc: Allow saving to HTML files

HtmlFile.save()
Snippet.saveToHtml()
Snippets.saveToHtml()
MainWindow.saveSnippetsToHtml()

write out the content of one or all snippets to corresponding files
in the /doc folder.

Currently there is no user interface for this, but I've tested that
it works correctly. To be useful this needs an alternative HTML output
for the HtmlFile class because then the navigation has to be done
*in* the page and not outside of it in the application.
oll-doc: Reflect html.py renaming in Eric project