wxDC clean-up

[Robert Roebling]

  Hi,

I've moved all the code doing the various kinds
of coordinate transforms in wxDC to wxDCBase.
There were about 10 slightly different versions
in every derived and this had to be corrected.
I hope I haven't broken too much, but I fear
that at least the Mac port will have to be
adapted (although easily).

I also removed SetSystemScale() on the way as
mentioned yesterday.

For those ports, where the upper left corner is
not 0,0, I introduced m_deviceLocalOriginX/Y
which is now used in the QD build of wxMac
instead of m_macLocalOrigin and also in the
Postscript DC (where 0,0 is at the bottom).
I seem to remember that the coordinate system
in OS/2 is also inverted, so you may want to
adapt that code. I have no idea about the 
status of that code.

It is still obvious that e.g. wxPostscriptDC
deriving from wxDC (including all its platform
dependent code) is terrible and needs to be
changed.

I probably broke RTL support on GTK+ on
the way.

  Robert