This builds upon the efforts started 2 commits back where \r\n is used for
all message text and \n is only used when the text is inside an
EolConvertingEditText widget.
Dynamically generate the CSS style for <pre> elements
for inclusion in the HTML <head> element when messages
are displayed.
This permits a user to change their font-family preference
for plain text messages and see the results immediately.
Obviously any old locally-stored messages that had their
font-family stored with them will continue to display using
that font-family, irrespective of the user's current
preference setting.
