allow user to add configurable header & footer

[avp90]

Here is an example for the extended header & footer configuration:

{
            index: 2,
            header: 'Share (EUR)',
            headerClasses: ['editable-table-header', 'text-center'],
            footer: '0',
            footerClasses: ['editable-table-footer', 'text-center', 'no-editor', 'sum-result'],
            name: 'share_eur',
            classes: flatten(['text-center', 'sum-col', 'percent-partial-value-row', additionalClasses]),
            isValid: isValidFloat,
            afterAdd: (value, cell) => updateSums(table, tableElement, tableDataFieldId, value, cell),
            afterChange: (value, cell) => updateSums(table, tableElement, tableDataFieldId, value, cell),
            convertOut: convertToFloat,
}

I've added header, headerClasses, footer, footerClasses to be able to use a such dynamic configuration for header & footer.