change to allow header and footer to be dynamic

[bsgriggs]

Hi there,
I'm not expecting a direct merge, but here is an example of allowing studio pro to dynamically allow a header and footer.

If you were to make this feature, I would recommend having an 'Header/Footer type' enum with

• "Static" (default value) exact same features as right now, to allow people to keep their current settings on upgrade
• "Dynamic" use boolean expression property types so Studio Pro can set whether or not to include a header/footer for that particular generation

[mgroeneweg]

I'm a little worried about unexpected results when the expression takes slightly longer to resolve. By then, body content may heve been rendered causing layout issues. That was for me the main reason to only use a fixed enum property