gui XRC: remove font's family=default

With the latest wxPython (3.0), the default value of font.family is the default font, but setting
it explicitly to "default" raises an error that the family is not known.

The fix is to just not set it to anything.
gui: update launch_xrced to more likely work with the default install

Make sure it's not needed to have the XRCed path directly in python path.
gui scalewindow: ensure no drawing happens at init

To initialise the buffer, just create it. That's easier, but also
ensure we don't draw, which causes problems with the latest wx 3.0:

type: C++ assertion "m_window" failed at ../src/gtk/dcclient.cpp(2043) in DoGetSize(): GetSize() doesn't work without window
gui: fix typo in comment
gui comp: fix buffer drawing to work with wx3.0

it seems using the clientDC early can break things, especially with GTK.
So use back the code as in the wiki, that works fine.

It fixes such error with wx3.0:
type: C++ assertion "m_window" failed at ../src/gtk/dcclient.cpp(2043) in DoGetSize(): GetSize() doesn't work without window
gui comp: minor clean ups