treewide: Subclass custom classes from GtkWidget

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

src/ui/contacts-box.ui

@@ -3,7 +3,7 @@
 <interface>
   <requires lib="gtk+" version="3.20"/>
   <requires lib="libhandy" version="0.0"/>
-  <template class="CallsContactsBox" parent="GtkBin">
+  <template class="CallsContactsBox" parent="GtkWidget">
     <property name="visible">True</property>
     <child>
       <object class="GtkScrolledWindow">

src/ui/history-box.ui

@@ -2,7 +2,7 @@
 <!-- Generated with glade 3.22.0 -->
 <interface>
   <requires lib="gtk+" version="3.22"/>
-  <template class="CallsHistoryBox" parent="GtkBin">
+  <template class="CallsHistoryBox" parent="GtkWidget">
     <child>
       <object class="GtkStack" id="stack">
         <property name="visible">True</property>

src/ui/in-app-notification.ui

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <requires lib="gtk+" version="3.20"/>
-  <template class="CallsInAppNotification" parent="GtkBin">
+  <template class="CallsInAppNotification" parent="GtkWidget">
     <property name="visible">False</property>
     <child>
       <object class="GtkRevealer" id="revealer">

src/ui/new-call-box.ui

@@ -3,7 +3,7 @@
 <interface>
   <requires lib="gtk+" version="3.22"/>
   <requires lib="libhandy" version="1.0"/>
-  <template class="CallsNewCallBox" parent="GtkBin">
+  <template class="CallsNewCallBox" parent="GtkWidget">
     <property name="visible">True</property>
     <child>
       <object class="GtkBox">