Allow borderimage .sci source to be a quoted filename

Allow borderimage .sci source to be a quoted filename

Previously, attempting to set the source property of a border image
via a .sci file to a quoted filename would fail.  This commit adds
support for quoted source filenames.

Task-number: QTBUG-20709
Change-Id: Ida54ef42bc07081457fd945bb279f3cc82c26e10
Reviewed-on: http://codereview.qt.nokia.com/2724
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Damian Jansen <damian.jansen@nokia.com>
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>