Don't assume that the last step of the XPath is an element name

Don't assume that the last step of the XPath is an element name

In order to find the number of items in the model, the model did a
second query on the document representing the result set.
So, suppose the user queried for //a/b/c, the result set would contain
all the 'c' elements from the input document.
In order to find the number of items in the result set, it did a second
query with the expression  "count(/dummy:items/c)", where 'c' was
extracted from the last step in the original XPath expression

For simple expressions, this worked fine.

However, if the last step had a predicate such as "//c/parent::b" it
didn't work.

The solution is to not filter *again* the last step when we query for
the count, since we know that all result items are direct children of
"dummy:items", and instead just execute the query
"count(dummy::items/*)".

This should also potentially improve performance.

Task-number: QTBUG-17588
Change-Id: Ib2fdf1ec8b91022df0597e089ad34d34b04428b0
Reviewed-by: Alan Alpert <aalpert@blackberry.com>