wxWidget compliance

Hi, new here.

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. Changed the code to better comply with wxWidgets, not using a lot of 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.