Fix warnings in sequence wrapper code

Fix warnings in sequence wrapper code

Previously, the sequence wrapper had unsigned int / signed int
comparisons (due to Qt container classes only allowing signed int
indexes (where negative indexes are invalid).

This commit ensures that unsigned indexes are bounds checked
appropriately, and also fixes a warning due to QString construction
from QByteArray.  Finally, it updates the documentation for sequences
to clarify the indexing semantics.

Change-Id: I4c6e133bef6e980a9ccb62ff15a70a5d41537ee3
Reviewed-by: Martin Jones <martin.jones@nokia.com>
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>