Also add wxObjArray::swap() for consistency with the other arrays

As with the previous commit, make this kind of arrays a bit more
comfortable to use as long as we can't avoid using them completely.

include/wx/dynarray.h

@@ -436,6 +436,8 @@ public:
 
     void Sort(CMPFUNC fCmp) { base::Sort(fCmp); }
 
+    void swap(wxBaseObjectArray& other) { base::swap(other); }
+
     // Provide a way to iterate over the stored objects using range-based for.
     class ObjectIterator
     {

tests/arrays/arrays.cpp

@@ -616,6 +616,15 @@ TEST_CASE("wxObjArray", "[dynarray]")
         CHECK( bars.GetCount() == 3 );
         CHECK( Bar::GetNumber() == 4 );
 
+        ArrayBars tmp;
+        bars.swap(tmp);
+        CHECK( bars.size() == 0 );
+        CHECK( Bar::GetNumber() == 4 );
+
+        bars.swap(tmp);
+        CHECK( bars.size() == 3 );
+        CHECK( Bar::GetNumber() == 4 );
+
         bars.RemoveAt(1, bars.GetCount() - 1);
 
         CHECK( bars.GetCount() == 1 );