Add support for UAProf header in HTTP requests

Each HTTP request sent from a mobile handset is usually required to
include a x-wap-profile header following the UAProf specification. The
value of this header is a URL that points to the location of a
document which specifies relevant capabilities of the phone, e.g.
supported network bearers, video formats or screen size. This change
defines a global string resource that holds this URL, and also adds
the necessary code in the web widget to include this header in HTTP
requests.