frontend 2.0: initial version

- auto refresh function
- light and dark theme
- monaco editor (vscode) with "apex" as syntax highlighting language
  * apex seems to work quite fine (better than no highlighting)
  * future possibility: own plantuml language syntax support
  * future possibility: autocomplete (to much work but maybe partial)
    - implemented example for `!theme ...`
    - implemented example for `<&icon>`
  * future possibility: code validation
    - implemented example for `@start...` and `@end...`:
      * should be the first or last command
      * should be of the some type (e.g. `@startyaml` and @endyaml)
      * should be used exactly once per document/diagram
- editor and preview is splitable into two windows like the
  "Extract window" functionality on
  (plantuml.com)[https://www.plantuml.com/plantuml]
- multi index / multi paging diagram support
- diagram can be displayed/rended as PNG, SVG, ASCII Art or PDF
- Ctrl+s download the PlantUML Code as code file (diagram.puml)
- Ctrl+, opens the settings and Esc closes the settings

src/test/java/net/sourceforge/plantuml/servlet/TestCharset.java

@@ -4,6 +4,8 @@ import java.io.IOException;
 import java.net.URL;
 import java.net.URLConnection;
 
+import net.sourceforge.plantuml.servlet.utils.WebappTestCase;
+
 
 public class TestCharset extends WebappTestCase {