Add shiny:restyle event for listening to instances where session() updates stylesheet(s).

Will be useful for real-time theming integration in flexdashboard https://github.com/rstudio/flexdashboard/pull/308

inst/www/shared/shiny.js

@@ -4033,6 +4033,10 @@ function _defineProperty(obj, key, value) { if (key in obj) { Object.definePrope
           sheet.disabled = true;
           if (browser.isIE) sheet.cssText = "";
           $(sheet.ownerNode).remove();
+          $(document).trigger({
+            type: 'shiny:restyle',
+            oldSheet: sheet
+          });
         };
 
         $.map(links, function (link) {

srcjs/output_binding_html.js

@@ -174,6 +174,9 @@ function renderDependency(dep) {
         sheet.disabled = true;
         if (browser.isIE) sheet.cssText = "";
         $(sheet.ownerNode).remove();
+        $(document).trigger({
+          type: 'shiny:restyle', oldSheet: sheet
+        });
       }
 
     }