splits export into the asking and doing part
extract test dummy editor and dummy viewer
refactor AskSaveOption to general DialogOption, implicit swing2dialogOption
ask overwrite on save and export

- new AskOverwrite mixin for the feature
- reorganized FileHandling
  - added chooseExportTarget (used chooseSaveTarget before
- pulled out AskSaveState to MockClientState for reuse
  - define implicit classes for the companion for specific clients