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)

docs/WebUI/import-export.md

@@ -0,0 +1,21 @@
+# Import and Export editable PlantUML Diagrams
+
+Similar to [draw.io](https://app.diagrams.net) it is possible to load and continue editing PlantUML diagram images.
+
+
+# Export a diagram
+
+Via the editor menu or Ctrl+S (or Meta+S in the case of a Mac) you can open the file save dialog.
+Here you can edit the file name, choose a file/diagram type and download the diagram.
+The default is to download the PlantUML code.
+
+![export](https://raw.githubusercontent.com/plantuml/plantuml-server/master/docs/WebUI/gifs/diagram-export.gif)
+
+
+# Import a diagram
+
+This feature is based on the PlantUML meta data which currently **support only PNG and SVG** diagrams.
+Besides a diagram image, you can of course also load a diagram code file.
+Moreover, because it is so nice and convenient, we also added a drag-and-drop feature.
+
+![import](https://raw.githubusercontent.com/plantuml/plantuml-server/master/docs/WebUI/gifs/diagram-import.gif)