Add support for <include> in workspace XML.

Note that this doesn't honor folder nesting (that is, you
can't include an XML file full of shortcuts inside a
<folder> node and expect them all to appear in that folder).
It's really just a way to concatenate/overlay multiple XML
files, making it easier to make a few tweaks/additions for
certain screen sizes or locales.

Bug: 10932771
Change-Id: I1fc29477d0717a8a7bc116f16729a955e6c625d5