Use RBuf instead of HBufC in qcore_symbian_p.h

Email from Mark Wilcox:

I had a quick look at those conversion functions. Generally, you guys would be much better off using RBuf rather than HBufC (which is one of the true evils of Symbian OS). I mentioned this to Espen at the smartphone show - you can replace most other descriptor use in Symbian code with RBuf and it simplifies almost everything.

There's an old paper on RBuf here:
https://developer.symbian.com/main/downloads/papers/RBuf/Introduction_to_RBuf_v1.0.pdf
Unfortunately the word hasn't been spread widely enough and people are still doing things the 'old' way through most of the ecosystem.

Your qt_TDesC2QStringL function makes 3 copies of the string data. You should be able to get that down to 1 using RBuf.