Support keyboard navigation in the Tags dialog

[bug #777234]

extensions/edit_metadata/data/ui/tag-chooser.ui

 <?xml version="1.0" encoding="UTF-8"?>
+<!-- Generated with glade 3.20.1 -->
 <interface>
-  <requires lib="gtk+" version="2.14"/>
-  <object class="GtkVBox" id="content">
+  <requires lib="gtk+" version="3.16"/>
+  <object class="GtkBox" id="content">
     <property name="visible">True</property>
     <property name="can_focus">False</property>
+    <property name="orientation">vertical</property>
     <property name="spacing">6</property>
     <child>
       <object class="GtkLabel" id="tag_label">
         <property name="visible">True</property>
         <property name="can_focus">False</property>
-        <property name="xalign">0</property>
+        <property name="halign">start</property>
+        <property name="hexpand">True</property>
         <property name="label" translatable="yes">T_ags:</property>
         <property name="use_underline">True</property>
       </object>

gthumb/gth-tags-entry.c

@@ -1078,7 +1078,6 @@ gth_tags_entry_init (GthTagsEntry *self)
 	GtkCellRenderer   *renderer;
 	GtkWidget         *menu_item;
 
-	gtk_widget_set_can_focus (GTK_WIDGET (self), TRUE);
 	gtk_style_context_add_class (gtk_widget_get_style_context (GTK_WIDGET (self)), GTK_STYLE_CLASS_COMBOBOX_ENTRY);
 	gtk_orientable_set_orientation (GTK_ORIENTABLE (self), GTK_ORIENTATION_VERTICAL);
 	gtk_box_set_spacing (GTK_BOX (self), 3);