Length of collapsoid

[michmech]

Allow the docspec author to set the length of collapsoids (the boxes of truncated text when an element is collapsed).

You can do this already by overriding Xonomy's default collapsoids and creating a custom "collapsoid" function instead. But this is too complex if all you want is to change the default length.

(Originally suggested by Euan Cameron, The University of Edinburgh).