Have an EVENT tag after STYLESHEETS in overall_header

In overall_header.html, an EVENT should be available after the STYLESHEETS tag.

This would allow style tags to be used after the included CSS files that can over-write styling from the linked CSS files if needed. It can also allow stuff to truly be inserted immediately before the closing head tag.

But we still need the EVENT before STYLESHEETS so extensions can use INCLUDECSS.

Perhaps have a new prepend event and some re-arranging:

<!-- EVENT overall_header_head_prepend -->

{$STYLESHEETS}

<!-- EVENT overall_header_head_append -->