Added icons for many menu items

CHANGES.md

docs/editor/global_comments.rst

docs/editor/icons/absatz.svg

docs/editor/icons/bold.svg

docs/editor/icons/edit.svg

docs/editor/icons/footnote.svg

docs/editor/icons/foreign.svg

docs/editor/icons/heading-level-1.svg

docs/editor/icons/heading-level-2.svg

docs/editor/icons/heading-level-3.svg

docs/editor/icons/heading-level-4.svg

docs/editor/icons/italic.svg

docs/editor/icons/nicht-ermittelt.svg

docs/editor/icons/page-begin.svg

docs/editor/icons/quelle.svg

docs/editor/icons/quote.svg

docs/editor/icons/sperrung.svg

docs/editor/icons/stellenkommentar.svg

docs/editor/icons/sup.svg

docs/editor/icons/vers.svg