TISTUD-6805 3.5.0 studio UI interface is not as clean as 3.4.0 studio

- clean up the look by removing jeeyul stuff. We do lose the ability to "hide" the empty editor stack, but we look much better again and can move away from using jeeyul's hacks.
- refactor css to use imports for common definitions
- Move the themes into our own themes plugin, remove the jeeyul plugin altogether