[gta02-core] upgrade to latest KiCad version (BZR 2358)

[Werner Almesberger]

We've been at ancient SVN revision 1923 for quite a while. There are
some goodies, particularly in pcbnew, that make it worth to upgrade.

I just went to the latest version, and things look quite good. It
complains about a few duplicate entries in gta02-core.lib, which
seem to be genuine (duplicate ANTENNA and such) and not too
difficult to fix.

Note that KiCad code and library have moved from SourceForge to
Launchpad, which implied a switch from Subversion to Bazaar. I've
updated the build instructions in GETTING-STARTED accordingly.

I also added build instructions for wxWidgets, in case the one
provided by the distribution doesn't contain the OpenGL support
KiCad now requires.

- Werner