Add loading spinner while restoring session

data/resources/ui/window.ui

@@ -8,14 +8,43 @@
         <property name="visible-child">login</property>
         <property name="transition-type">crossfade</property>
         <child>
-          <object class="Login" id="login" />
+          <object class="Login" id="login"/>
         </child>
         <child>
           <object class="GtkStack" id="sessions">
             <property name="transition-type">crossfade</property>
           </object>
         </child>
+        <child>
+          <object class="GtkWindowHandle" id="loading_page">
+            <property name="child">
+              <object class="GtkBox">
+                <property name="orientation">vertical</property>
+                <child>
+                  <object class="GtkHeaderBar">
+                    <property name="show-title-buttons">True</property>
+                    <style>
+                      <class name="flat"/>
+                    </style>
+                  </object>
+                </child>
+                <child>
+                  <object class="GtkSpinner">
+                    <property name="spinning">True</property>
+                    <property name="valign">center</property>
+                    <property name="halign">center</property>
+                    <property name="vexpand">True</property>
+                    <style>
+                      <class name="session-loading-spinner"/>
+                    </style>
+                  </object>
+                </child>
+              </object>
+            </property>
+          </object>
+        </child>
       </object>
     </child>
   </template>
 </interface>
+

src/window.rs

@@ -25,6 +25,8 @@ mod imp {
         pub login: TemplateChild<Login>,
         #[template_child]
         pub sessions: TemplateChild<gtk::Stack>,
+        #[template_child]
+        pub loading_page: TemplateChild<gtk::WindowHandle>,
     }
 
     #[glib::object_subclass]
@@ -134,7 +136,7 @@ impl Window {
                 }
 
                 if n > 0 {
-                    self.switch_to_sessions_page();
+                    self.switch_to_loading_page();
                 }
             }
             Err(error) => warn!("Failed to restore previous sessions: {:?}", error),
@@ -181,6 +183,13 @@ impl Window {
         priv_.main_stack.set_visible_child(&priv_.sessions.get());
     }
 
+    pub fn switch_to_loading_page(&self) {
+        let priv_ = imp::Window::from_instance(self);
+        priv_
+            .main_stack
+            .set_visible_child(&priv_.loading_page.get());
+    }
+
     pub fn switch_to_login_page(&self) {
         let priv_ = imp::Window::from_instance(self);
         priv_