[QtScript/JavaScriptCore] add UString(QString) constructor and operator QString()

Patch: http://gitorious.org/+qtwebkit-developers/qtwebkit/qtwebkit-qtscript-integration/commit/4b9d9aea2a007f0e9fed4221f9401b627405d55f

Reason: As convenience, avoid having to explicitly call conversion functions.
There are similar conversions other places in WebKit, e.g. KURL <-> QUrl, IntRect <-> QRect.

IMO we should get rid of it, since we actually do care where this conversion is done (for performance reasons we want to avoid conversion back and forth).
So long as the goal is that the QtScript API should not access any of the internal types of JSC anyway, this patch has no place.