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>

plugins/provider/sip/sip-account-widget.ui

@@ -8,8 +8,6 @@
         <property name="orientation">vertical</property>
         <child>
           <object class="AdwHeaderBar" id="header_add">
-            <property name="title" translatable="yes">Add Account</property>
-            <property name="show-close-button">True</property>
             <child type="end">
               <object class="GtkButton" id="login_btn">
                 <property name="use-underline">True</property>
@@ -28,8 +26,6 @@
         <child>
           <object class="AdwHeaderBar" id="header_edit">
             <property name="visible">True</property>
-            <property name="show-close-button">True</property>
-            <property name="title" translatable="yes">Manage Account</property>
             <child type="end">
               <object class="GtkButton" id="apply_btn">
                 <property name="use-underline">True</property>