Add import/export

[marcelklehr]

see https://developer.mozilla.org/en-US/docs/Web/HTTP/data_URIs

• [x] use mime types as document types / ot types / editor types?
• [x] add an import/export delegator
  • [x] all editors should add a default import option for their document.type here (plugins may add additional options)
  • [x] all editors should add a default export option for their document.type here (plugins may add additional options)
• [x] export ui: pass down available export options in config and render by ui-importexport
• [x] import ui
  • [x] ui: make #editor a filedrop target
  • [x] ui: add an import navbutton