ui: Remove explicit can-focus

Glade misleads us into thinking we need these, but the
property is somewhat confusing in GTK3. In general we don't
want `can-focus` to be true for containers, as that will
break keynav for the children. For non-containers, the
defaults are appropriate.

Instead of manually setting `can-focus`, we should just
use the default values. GTK has sane defaults.

Fixes https://gitlab.gnome.org/GNOME/calls/-/issues/366

src/ui/history-box.ui

@@ -16,16 +16,13 @@
 
     <child>
       <object class="GtkScrolledWindow">
-        <property name="can_focus">False</property>
         <property name="visible">True</property>
         <property name="hscrollbar-policy">never</property>
         <child>
           <object class="HdyClamp">
-            <property name="can_focus">False</property>
             <property name="visible">True</property>
             <child>
               <object class="GtkListBox" id="history">
-                <property name="can_focus">False</property>
                 <property name="visible">True</property>
                 <property name="margin">12</property>
                 <property name="valign">start</property>