[FIX] web: display warning message for o2m modal

Be a little less strict when not showing o2m list view warning so if the
o2m is not editable (a modal is open instead of editing inline) we still
show warnings.

Also have warnings over modal overlays so they can be used for modals.

opw-702333

addons/web/static/src/js/views/form_relational_widgets.js

@@ -1070,8 +1070,10 @@ var One2ManyListView = X2ManyListView.extend({
             this._dataset_changed = false;
         });
 
-        this.on('warning', this, function(e) { // In case of a one2many, we do not want any warning which comes from the editor
-            e.stop_propagation();
+        this.on('warning', this, function(e) { // In case of editable list view, we do not want any warning which comes from the editor
+            if (this.editable()) {
+                e.stop_propagation();
+            }
         });
     },
     do_add_record: function () {

addons/web/static/src/less/notification.less

@@ -5,7 +5,7 @@
     max-width: 100%;
 
     .o-position-absolute(@navbar-height, 0px);
-    z-index: 50;  // same as debug manager. Fixme: expose a variable
+    z-index: 1050;  // above the modals. Fixme: expose a variable
 
     .o_notification {
         padding: 0;