refactor: re-layput project

- move assets (raw jsons, icons, meshes and i18n data) into the root of repo.
- move script into the root of repo.
- modify blender manifest according to this changes.
- optimize gitignore.

i18n/.gitignore

@@ -0,0 +1,4 @@
+# Do not include intermidiate translation template (POT)
+# Only keep the final merged POT file.
+*.pot
+!blender.pot