[FIX] web_editor: mark snippets which can't be dropped in the page

The system was lost with the editor refactoring which came with saas-10
and commit bf39884a5c.
Indeed the show() method of the BuildingBlock class was removed as the
widget is now always showed but the show_blocks() method it called was
not removed (and thus was not called anymore).

As a fix, the show_blocks method is now called again, on snippets
initializations and each time a snippet is dropped or removed.

addons/web_editor/static/src/js/snippets.editor.js

@@ -356,6 +356,9 @@ var BuildingBlock = Widget.extend({
         });
 
         self.make_snippet_draggable(self.$snippets);
+
+        this.show_blocks();
+        this.$el.on("snippet-dropped snippet-removed", this.show_blocks.bind(this));
     },
 
     cover_target: function ($el, $target) {
@@ -1074,6 +1077,8 @@ var Editor = Class.extend({
         $(".note-control-selection").hide();
         $('.o_table_handler').remove();
 
+        this.buildingBlock.$el.trigger("snippet-removed");
+
         return false;
     },