gtkplacessidebar: implement libcloudproviders support

Add integration of the libcloudproviders DBus API to the
GtkPlacesSidebar by showing name and sync status of the cloud providers.
The exported menu is rendered as a GtkPopover.

The sidebar will be updated if the list of cloudproviders changes e.g.
by adding or removing an account. If any cloud provider changes detailed
information like sync status only the individual sidebar row gets
updated.

Co-authored-by: Carlos Soriano <csoriano@gnome.org>

https://bugzilla.gnome.org/show_bug.cgi?id=786123

gtk/ui/gtksidebarrow.ui

@@ -22,8 +22,8 @@
               <object class="GtkBox">
                 <property name="visible">1</property>
                 <child>
-                  <object class="GtkImage" id="icon_widget">
-                    <property name="visible">1</property>
+                  <object class="GtkImage" id="start_icon_widget">
+                    <property name="visible">True</property>
                     <style>
                       <class name="sidebar-icon"/>
                     </style>
@@ -39,6 +39,17 @@
                     </style>
                   </object>
                 </child>
+                <child>
+                  <object class="GtkImage" id="end_icon_widget">
+                    <property name="visible">False</property>
+                    <property name="hexpand">True</property>
+                    <property name="halign">end</property>
+                    <property name="valign">center</property>
+                    <style>
+                      <class name="sidebar-icon"/>
+                    </style>
+                  </object>
+                </child>
                 <child>
                   <object class="GtkButton" id="eject_button">
                     <property name="visible">1</property>
@@ -67,4 +78,14 @@
       </object>
     </child>
   </template>
+  <!-- We need it to not make the row smaller when the eject button is hidden -->
+  <object class="GtkSizeGroup">
+    <property name="mode">vertical</property>
+    <widgets>
+      <widget name="eject_button"/>
+      <widget name="label_widget"/>
+      <widget name="start_icon_widget"/>
+      <widget name="end_icon_widget"/>
+    </widgets>
+  </object>
 </interface>