ui: Remove can_focus from GTK builder file definitions

GTK widgets set the default for this as needed, and some incorrect
values for this have caused issues with keyboard focus.

Fixes #1059

ui/accounts_editor.ui

@@ -3,7 +3,6 @@
 <interface>
   <requires lib="gtk+" version="3.20"/>
   <template class="AccountsEditor" parent="GtkDialog">
-    <property name="can_focus">False</property>
     <property name="modal">True</property>
     <property name="default_width">360</property>
     <property name="default_height">450</property>
@@ -13,11 +12,9 @@
     </child>
     <child internal-child="vbox">
       <object class="GtkBox">
-        <property name="can_focus">False</property>
         <property name="orientation">vertical</property>
         <child internal-child="action_area">
           <object class="GtkButtonBox">
-            <property name="can_focus">False</property>
             <property name="layout_style">end</property>
             <child>
               <placeholder/>
@@ -35,11 +32,9 @@
         <child>
           <object class="GtkOverlay" id="notifications_pane">
             <property name="visible">True</property>
-            <property name="can_focus">False</property>
             <child>
               <object class="GtkStack" id="editor_panes">
                 <property name="visible">True</property>
-                <property name="can_focus">False</property>
                 <property name="transition_type">slide-left-right</property>
                 <signal name="notify::visible-child" handler="on_pane_changed" swapped="no"/>
                 <child>