add html header and footer tags for layer and class blocks in legend templates

[schut@…]

In addition to the general legend html header and footer tags [leg_header_html]
and [leg_footer_html] I would like to see the same kind of tags for the separete
legend components, especially for layer and class but then why not also for
group too. So we would get [leg_group_header_html], [leg_layer_header_html],
[leg_class_header_html], their closing equivalents, and the corresponding footer
tags. Code that is between these [header] [/header] tags will be placed before
the parsed group/layer/class block, code in the footer tags after the
corresponding block, just like code within the generic [leg_header_hmtl] and
[leg_footer_html] tags is placed before and after the entire parsed legend block.
To illustrate one situation where this would come in handy: if you want to place
all layers of a group in one <div id=layerblock> </div> block to have it
rendered using css id's/classes.