Add Shift+Arrow keys for expand/collapse in XML editor.

Fixes #55

(cherry picked from commit 43c4e6e8)

keys/keys.xml

@@ -471,12 +471,20 @@ supposed to work the same as corresponding regular keys.</para>
         <listitem>
           <para>
             <keycap>+</keycap>
+            <keycombo>
+              <keycap function="shift">Shift</keycap>
+              <keycap>Right arrow</keycap>
+            </keycombo>
             <action>expand direct children of the currently selected element</action>
           </para>
         </listitem>
         <listitem>
           <para>
             <keycap>-</keycap>
+            <keycombo>
+              <keycap function="shift">Shift</keycap>
+              <keycap>Left arrow</keycap>
+            </keycombo>
             <action>collapse the currently selected element</action>
           </para>
         </listitem>