Use Wx3.0 instead of bundled 2.9.3 if present in the system.

wxWidgets developers manage to point out fixes for the breaks in compatibility with wxWidgets 2.9.5.
http://trac.wxwidgets.org/ticket/15529

With this pull request if wxWidgets 3.0 is present in the system, than it will be used instead of bundled 2.9.3. On Linux it works great on wxWidgets 3.0.0-r1 without pink dots regression: #288 and still provide backward compatibility with wxWidgets 2.9.3.

I publish this pull request for testing purposes. I hope that when wxWidgets 3.0 comes out it will be working great with this changes and then we can merge it to the master.