[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 ;)

css/main.css

@@ -239,3 +239,16 @@ sup {
   border-spacing: 0;
   border-collapse: separate;
 }
+
+/**
+ * Container Definitions.
+ */
+
+.region .container {
+  display: flex;
+  flex-direction: column;
+}
+
+.region .container.hidden {
+  display: none;
+}