simplify building tab stops in defined styles

When building the 'tab-stops' style rule, build a list of tab stop
definitions and then use join(), rather than building the string
directly with the requisite test for first element whether or not to add
the separator.
make inline style rules mutable

Maintain inline style rules as a list rather than joining them
immediately. Modify extract_style_rules to append to an existing list.
This will allow extract_style_rules to edit existing rules, which is
needed to extract tab stops into a single rule, without other
non-trivial modification of the code to deal with the fact that we have
an XML stream and not a DOM.
parse tab stops in inline styles

Add handling of style:tab-stops and style:tab-stop to
extract_style_rules, leveraging our new ability to modify existing rules
so that the former can create the rule with value 'none', and the latter
can replace that value or add additional values as appropriate.
un-convert HTML entities

Turn some HTML entities, that apparently have been converted to their
normal character form, back into HTML entities in order to avoid
ambiguity with style markup.