Merged PR 167: Add Flex and fix bugs (read desc)

Note: The branch name does not fit the new features.

- Implement Flex with simplex
- Enable rigid body by default (removed IsRigidBody property) <=== possibly a bad idea
- Sort children in add and update properties
- Implement MaxWidth
- Add more docs

Fixes :
- .env.production url
- Symbols: not blocking the linked container when the parent is moving

src/Enums/AddMethod.ts

@@ -2,9 +2,11 @@
  * Add method when creating a container
  * - Append will append to the last children in list
  * - Insert will always place it at the begining
+ * - Replace will remove the selected container and insert a new one
  * (default: Append)
  */
 export enum AddMethod {
   Append,
-  Insert
+  Insert,
+  Replace // TODO: Implement this
 }