Add rvalue-ref qualified {QString,QByteArray}::{simplified,trimmed}

Of the const overloads that return a QString or a QByteArray, this is
one that gains the most benefit. It happens often in constructs like:

    QByteArray s = x.readLine().trimmed();

After this change, 41 out of 103 calls to trimmed become rvalue in Qt
and 272 out of 441 in Qt Creator. For simplified, the numbers are 27 out
of 69 in Qt and 10 out of 19 in Qt Creator.

Other candidates are left, right, and mid, but there are exactly zero
uses of left, right and mid on an xvalue QString or QByteArray in Qt.

I'm being lazy and using qstring_compat.cpp to store the QByteArray
compat methods.

Change-Id: I4e410fc1adc4c761bb07cc3d43b348a65befa9f6
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>