Added WX_UTF8 macro

  The WX_UTF8 macro is primarily for convenience so you can input a
UTF-8 string into a wxWidgets function without having to do
wxString(string, wxConvUTF8) each time.

obs/wx-wrappers.hpp

@@ -23,6 +23,8 @@
 
 #include <vector>
 
+#define WX_UTF8(str) wxString(str, wxConvUTF8)
+
 struct gs_window;
 
 gs_window WxToGSWindow(const wxWindow *window);