Implement V8 util for converting String to QString.

[TIMOB-13638]
Switch to using V8ValueToQString().

[TIMOB-13638]
Remove TiObject::getStringFromValue().

[TIMOB-13638]
Remove NativeControlObject::getString().

Use V8ValueToQString() in V8Utils instead.

[TIMOB-13638]
Merge branch 'TIMOB-13638'