New: better memory management (GC for return values)

[Polos Ruetz]

The places where memory could be leaked are removed now (e.g. by
adding garbage collection for return values).
So, practically speaking, you only have to remember 1 rule:

- If you need a local Qt object, use QLET instead of QNEW (e.g.
QDialog, QRegExp).

The analogy of LET* and QLET should be sufficient to remember this rule.
Please see doc/Notes.htm in the sources for more details.

Paul