malcontent-control: Replace border-width by margins

The 'border-width' property does not exist in GTK4 anymore, so
replace it with margin-* and in one case just remove it entirely.

malcontent-control/carousel.ui

@@ -9,7 +9,10 @@
     <child>
       <object class="GtkOverlay">
         <property name="hexpand">True</property>
-        <property name="border_width">16</property>
+        <property name="margin-top">16</property>
+        <property name="margin-bottom">16</property>
+        <property name="margin-start">16</property>
+        <property name="margin-end">16</property>
         <child>
           <object class="GtkStack" id="stack">
             <property name="transition_duration">400</property>
@@ -24,7 +27,10 @@
             <child>
               <object class="GtkBox">
                 <property name="orientation">GTK_ORIENTATION_HORIZONTAL</property>
-                <property name="border_width">12</property>
+                <property name="margin-top">12</property>
+                <property name="margin-bottom">12</property>
+                <property name="margin-start">12</property>
+                <property name="margin-end">12</property>
                 <child>
                   <object class="GtkButton" id="go_back_button">
                     <property name="visible">False</property>

malcontent-control/main.ui

@@ -34,7 +34,6 @@
             <child>
               <object class="GtkBox">
                 <property name="orientation">vertical</property>
-                <property name="border_width">0</property>
                 <child>
                   <object class="MctUserSelector" id="user_selector">
                     <property name="user-manager">user_manager</property>