dirdiff.ui: Make copy labels more explicit (bgo#737373)

Author: kaiw

data/ui/dirdiff.ui

@@ -14,7 +14,7 @@
     </child>
     <child>
       <object class="GtkAction" id="DirCopyLeft">
-        <property name="label" translatable="yes">Copy _Left</property>
+        <property name="label" translatable="yes">Copy to _Left</property>
         <property name="tooltip" translatable="yes">Copy to left</property>
         <property name="stock_id">gtk-go-back</property>
         <property name="is_important">True</property>
@@ -24,7 +24,7 @@
     </child>
     <child>
       <object class="GtkAction" id="DirCopyRight">
-        <property name="label" translatable="yes">Copy _Right</property>
+        <property name="label" translatable="yes">Copy to _Right</property>
         <property name="tooltip" translatable="yes">Copy to right</property>
         <property name="stock_id">gtk-go-forward</property>
         <property name="is_important">True</property>