wxWidgets 3.x support 

Hi, 
I'm new to here, but have been using wxMaxima for quite some time. 

I compiled wxMaxima on ubuntu 14.04 using wxWidgets 3.1 and got some runtime errors. I guess the level of assertions for my build was higher than usual (wxWidgrts 3.x is stricter). Changed the code to better comply with newer wxWidgets, not using a lot of now deprecated variants of functions.

Also added a #define-guard to match enabling of menu item "menu_copy_as_bitmap" to when it is defined. This fixes a runtime error on ubuntu.

Added a wxWidgets include file which include <wx/wx.h> and fixes needed inconsistencies. 

Please let me know if I need to change something for this to be acceptable. It seems to work for me with wxWidgets versions 2.8.4 and the latest development version.

Cheers, Johan