1. Make it take string view: helps to avoid extra string allocation
e.g. in desktop::jsonToPropertyValuesVector, and will help more,
when C++26 stringstream ctor taking string view is available.
2. Factor out a function taking boost::property_tree::ptree, making
implementation simpler for [][]com.sun.star.beans.PropertyValue and
[]com.sun.star.beans.PropertyValue, without writing a child node to
a JSON string, and parsing it again.
Change-Id: I16ac2641633ea67a7c9c054c9df09a790500e6fb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175361
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>
