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

public/Interfaces.d.ts

@@ -38,7 +38,7 @@ declare interface IProperties extends React.CSSProperties {
   parentId: string | null
   x: number
   y: number
-  isRigidBody: boolean
+
   XPositionReference?: XPositionReference
 }