Add support for QUrl types to V4

Add support for QUrl types to V4

Extended the V4 instruction set with instructions to
`fast convert' url registers to string and bool registers
and `resolve' urls using QDeclarativeContext::resolvedUrl.

Also, made IR::UrlType a special `string' type. It's a little trick
to ensure that the compiler will generate correct conversions for
the binary expressions.

Change-Id: Ibc9e5b99302bd513f0cc52b598a1b198b11d4d30
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>