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)

.vscode/settings.json

@@ -12,6 +12,7 @@
         "Lalloni",
         "monaco",
         "plantuml",
+        "puml",
         "Roques",
         "servlet",
         "servlets",
@@ -20,5 +21,6 @@
         "undock",
         "utxt"
     ],
-    "cSpell.allowCompoundWords": true
-}
+    "cSpell.allowCompoundWords": true,
+    "svg.preview.background": "transparent"
+}