Eliminate -Wcast-qual warnings with GCC and Clang

Use const_cast, mutable, and various other changes to avoid -Wcast-qual

include/wx/dynarray.h

@@ -391,7 +391,7 @@ public:
 
     void Insert(const T* pItem, size_t uiIndex)
     {
-        base::insert(this->begin() + uiIndex, (T*)pItem);
+        base::insert(this->begin() + uiIndex, pItem);
     }
 
     void Empty() { DoEmpty(); base::clear(); }