[Feature Request] Allow to make an display order of module position in config (#3762)

My proposal for #3761 

* I use `flex` box for create region container
* I have moved `container` definition to `main.css` (for css tunning,
maybe it can be useful)
* I create `order` in module config for define module order display from
position

# Advantage:

We don't have to move module config in another place in array of modules

# Another advantage that i did'nt think:

We can change dynamicaly module order of a container:

- So, in this case, for a module config builder, (I won't mention the
name because otherwise I'll get angry...)
It can be usefull for a config preview (before saving config)
--> just change style css `order` module value for see preview

- Or, change `order` value in dev console for testing

# Disadvantages

I don't see any ;)

js/loader.js

@@ -108,7 +108,8 @@ const Loader = (function () {
 				header: moduleData.header,
 				configDeepMerge: typeof moduleData.configDeepMerge === "boolean" ? moduleData.configDeepMerge : false,
 				config: moduleData.config,
-				classes: typeof moduleData.classes !== "undefined" ? `${moduleData.classes} ${module}` : module
+				classes: typeof moduleData.classes !== "undefined" ? `${moduleData.classes} ${module}` : module,
+				order: (typeof moduleData.order === "number" && Number.isInteger(moduleData.order)) ? moduleData.order : 0
 			});
 		});