add import and export diagram

- 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)

src/main/webapp/assets/actions/dock.svg

@@ -0,0 +1 @@
+<svg height="512" width="512" xmlns="http://www.w3.org/2000/svg"><path d="M384 224v184a40 40 0 01-40 40H104a40 40 0 01-40-40V168a40 40 0 0140-40h167.48" fill="none" stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="32"/><path fill="none" stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="32" d="M216 184v112h112m-104-8L440 72"/></svg>