IntelliJ Themes:

- added themes list to demo (and testing) apps
- added some popular open-source IntelliJ themes to demo
  (downloaded latest .theme.json from source repos;
  see themes.properties for source repo URLs)

CHANGELOG.md

@@ -1,6 +1,11 @@
 FlatLaf Change Log
 ==================
 
+## Unreleased
+
+- Support using IntelliJ platform themes (.theme.json files).
+
+
 ## 0.18
 
 - TextField and TextArea: Do not apply minimum width if `columns` property is