oll-doc: Sort tags/authors (again)

This had been removed unintentionally.
oll-doc: factor out MainWindow.showSnippet()
oll-doc: reorder standard header fields
oll-doc: Make multiline strings strings, not lists

A multiline string can still be treated differenty thanks
to the \n in it. However, I want to be able to treat 'real'
lists differently, so only they are explicitly created.
oll-doc: Add initial HTML doc viewer

Add a QWebView widget and generate HTML documentation
from the header fields. Currently this uses css style sheets
without defining them. This is a TODO.
line-break-marks: Fix typo in usage example file