treewide: Adapt to GtkHeaderBar API changes

https://docs.gtk.org/gtk4/migrating-3to4.html#adapt-to-gtkheaderbar-and-gtkactionbar-api-changes

- `show-close-button` replaced with `show_title_buttons` wherever
  applicable, respecting new default value of `TRUE`
- `title` removed and managed by setting window title if needed
    - NOTE: I'm not 100% sure if the title logic here matches the GTK3
      version. I also just removed the SIP account widget titles
      altogether, since it's I couldn't figure out where the widget gets
      placed.

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

src/ui/main-window.ui

@@ -14,8 +14,6 @@
           <object class="AdwHeaderBar">
             <property name="visible">True</property>
             <property name="centering_policy">strict</property>
-            <property name="show_close_button">True</property>
-            <property name="title" bind-source="CallsMainWindow" bind-property="title" bind-flags="sync-create"/>
             <child type="title">
               <object class="AdwViewSwitcherTitle" id="title_switcher">
                 <property name="visible">True</property>
@@ -90,8 +88,8 @@
     <child type="titlebar">
       <object class="AdwHeaderBar">
         <property name="visible">True</property>
-        <property name="show-close-button">False</property>
-        <property name="title" translatable="yes">USSD</property>
+        <property name="show-start-title-buttons">False</property>
+        <property name="show-end-title-buttons">False</property>
         <child type="start">
           <object class="GtkButton" id="ussd_cancel_button">
             <property name="has-default">True</property>