A GridField manipulator that provides HTML for the header/footer rows, or f or before/after the template.
Returns a map where the keys are fragment names and the values are pieces of HTML to add to these fragments.
Here are 4 built-in fragments: 'header', 'footer', 'before', and 'after', but components may also specify fragments of their own.
To specify a new fragment, specify a new fragment by including the text "$DefineFragment(fragmentname)" in the HTML that you return.
Fragment names should only contain alphanumerics, -, and _.
If you attempt to return HTML for a fragment that doesn't exist, an exception will be thrown when the GridField is rendered.