FormLegend: refactor to be part of FormFieldset

[jbadan]

Description

FormLegend doesn't need to be it's own component, we should make a prop for FormFieldset that populates the legend.

Question: can a <legend> ever be used on it's own? If that answer is no:

1. Delete FormLegend https://github.com/SAP/fundamental-react/blob/master/src/Forms/FormLegend.js
2. Move FormLegend functionality into FormFieldset via prop eg:

<FormFieldset legend='my localized legend text' />

<FormFieldset legend={(<h3>my localized text</h3>)} />

Implement according to spec: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/legend ^ might require a change for underlying html in fundamental-styles

Versions

fundamental-react: 0.13.0

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