Implement a modal system

I decided to create my own and not to use a community package, because
1. it's straightforward
2. it's better integrated with our others libs such as EscapeActions
3. monitoring third-party packages evolutions (eg, CSS changes) is a
   lot of work.

This is basically the same rationale than for our other generic UI
components such as the Popup/Popover.

This commit also slightly modify the general layout to remove
unnecessary wrapper DOM nodes.

client/lib/escapeActions.js

@@ -13,6 +13,7 @@ EscapeActions = {
     'popup-close',
     'inlinedForm',
     'detailsPane',
+    'modalWindow',
     'multiselection',
     'sidebarView'
   ],