Allow user specified line splitting in titles

Titles can be split at specified points with '\n' within the title
string. Titles segments which are still too long will still be split
over multiple lines in the normal way.
Differentiates title style classses

The main plot title is given an additional class (plot_title) to
differentiate it from from the axis titles, so the it could for example
be styled in boldface using CSS.
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