linux: use wx3.0 by default

+ Wx3.0 fixes various issue on linux (better sizing of box)
+ Debian doesn't provide wx2.8 anymore. Ubuntu will probably follow soon (close issue #342)

If you want to use wx2.8:
* Either you remove wx3.0 from your system
* Either you use the cmake option -DWX28_API=TRUE

Please don't hesitate to report any regresions.