Update to Catch v2

Use Catch2 public header instead of using internal Catch v1 headers: we
can do this now because Catch2 supports (albeit in a rather ugly way)
being used with PCH.

tests/allheaders.cpp

@@ -52,7 +52,7 @@
     #define __USE_W32_SOCKETS
 #endif
 
-#include "catch.hpp"
+#include "catch2/catch.hpp"
 
 #if defined(__WXQT__)
     // Include this one before enabling the warnings as doing it later, as it