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/new-call-header-bar.ui

@@ -2,7 +2,6 @@
 <interface>
   <requires lib="gtk+" version="3.22"/>
   <template class="CallsNewCallHeaderBar" parent="GtkHeaderBar">
-    <property name="can_focus">False</property>
     <property name="show-close-button">True</property>
     <property name="title" translatable="yes">New Call</property>
     <property name="visible">True</property>
@@ -34,7 +33,6 @@
     </child>
     <child type="title">
       <object class="GtkStackSwitcher" id="stack_switcher">
-        <property name="can_focus">False</property>
         <property name="visible">True</property>
       </object>
     </child>