LibGUI: Extend mimic pressed across keyboard shortcuts for buttons

Primary motivation for this was to get a visual indication in the
browser for Ctrl-R refresh. This extends what ForLoveOfCats had done
for calculator button shortcuts across all buttons with shortcuts.

When an action is triggered without an activator each associated button
will be set as mimic pressed.

Userland/Libraries/LibGUI/Action.cpp

@@ -131,6 +131,12 @@ void Action::activate(Core::Object* activator)
         }
     }
 
+    if (activator == nullptr) {
+        for_each_toolbar_button([](auto& button) {
+            button.set_mimic_pressed(true);
+        });
+    }
+
     on_activation(*this);
     m_activator = nullptr;
 }