use db to mark keys which can certify

OpenPGP-Keychain/res/layout/key_list_child_item.xml

@@ -63,6 +63,12 @@
             android:paddingBottom="2dip"
             android:paddingTop="2dip" >
 
+            <ImageView
+                android:id="@+id/ic_certifyKey"
+                android:layout_width="wrap_content"
+                android:layout_height="wrap_content"
+                android:src="@drawable/certify_small" />
+
             <ImageView
                 android:id="@+id/ic_encryptKey"
                 android:layout_width="wrap_content"
@@ -93,4 +99,4 @@
             android:textAppearance="?android:attr/textAppearanceSmall" />
     </LinearLayout>
 
-</LinearLayout>
+</LinearLayout>