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/PropertyType.ts

@@ -0,0 +1,22 @@
+
+/**
+ * Describe the type of the property.
+ * Used for the assignation in the OnPropertyChange function
+ * See ContainerOperations.ts's OnPropertyChange
+ */
+export enum PropertyType {
+  /**
+   * Simple property: is not inside any object: id, x, width... (default)
+   */
+  SIMPLE,
+
+  /**
+   * Style property: is inside the style object: stroke, fillOpacity...
+   */
+  STYLE,
+
+  /**
+   * Margin property: is inside the margin property: left, bottom, top, right...
+   */
+  MARGIN,
+}