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/resource/preview.jsp

@@ -32,7 +32,7 @@
             id="btn-settings"
             class="btn-settings"
             type="image"
-            src="assets/settings.svg"
+            src="assets/actions/settings.svg"
             alt="settings"
             onclick="openSettings();"
           />
@@ -40,7 +40,7 @@
             id="btn-undock"
             class="btn-dock"
             type="image"
-            src="assets/undock.svg"
+            src="assets/actions/undock.svg"
             alt="undock"
             onclick="undock();"
           />
@@ -48,7 +48,7 @@
             id="btn-dock"
             class="btn-dock"
             type="image"
-            src="assets/dock.svg"
+            src="assets/actions/dock.svg"
             alt="dock"
             onclick="window.close();"
             style="display: none;"