decrypt: Set a window title.

Assign a title to the decrypt/verify dialog.

src/yki-decrypt.ui

@@ -18,6 +18,7 @@
   <object class="GtkDialog" id="dlgDecrypt">
     <property name="can_focus">False</property>
     <property name="border_width">5</property>
+    <property name="title" translatable="yes">Yorkie - Decrypt/Verify</property>
     <property name="type_hint">dialog</property>
     <child internal-child="vbox">
       <object class="GtkVBox" id="dialog-vbox1">