- export diagram
* add a diagram export dialog where you can choose the file name
and download type (code, png, pdf, ...)
* set default download type to code
* open file save dialog via menu or Ctrl+S (or Meta+S for Mac)
- import diagram
* similar to [draw.io](https://app.diagrams.net)
* open a PlantUML diagram image, use metat data to get diagram code
and load this diagram (Note: meta data is currently only supported
by PNG and SVG diagram files)
* support drag&drop
* add diagram import dialog
- since three are now multiple options/action -> create a little
editor menu
- add documentation (including gif examples)
