[ALOY-1052] Alloy: Support use of L in XML proxy elements

Given the following index.xml:

<Alloy>
  <Window class="container">
    <Label text="L('foo')" top="100" />
    <TabbedBar id="planning_columns" top="200">
      <Labels>
        <Label title="L(foo)" />
        <Label title="L('foo')" />
        <Label titleid="foo" />
        <Label class="foo" />
      </Labels>
    </TabbedBar>
  </Window>
</Alloy>

Only the last <Label> of the <TabbedBar> , which has the title set using L('foo') in TSS works. The first 2 show L(foo) as titel and the 3rd one is empty, which is expected as [BarItemType](http://docs.appcelerator.com/titanium/latest/#!/api/BarItemType) doesn't support it (which of course it should, but thats a different story/ticket). I'd expect L() to work in proxy elements, just like they do in the regular elements like the first <Label> above. See the attached example project and screenshot.