Cache wxDisplayImpl object for faster access

Avoid a heap allocation on every wxDisplay creation by caching the
wxDisplayImpl objects once they're created.

Currently we never invalidate the cache, but we should add a way to do
it in the future.

This speeds up wxDisplay::GetGeometry() benchmark by a factor of 4.

include/wx/display.h

@@ -50,7 +50,6 @@ public:
 
     // dtor is not virtual as this is a concrete class not meant to be derived
     // from
-    ~wxDisplay();
 
 
     // return the number of available displays, valid parameters to