Allow cloning client data stored in wxGrid attributes etc

Add wxSharedClientDataContainer class storing ref-counted client data
and use it instead of plain wxClientDataContainer in wxGridCellAttr,
wxGridCellEditor and wxGridCellRenderer classes.

This allows to keep the same client data associated with many grid cells
without having to make many copies of it.

tests/controls/gridtest.cpp

@@ -1729,6 +1729,21 @@ TEST_CASE_METHOD(GridTestCase, "Grid::CellAttribute", "[attr][cell][grid]")
         CHECK_ATTR_COUNT( 0 );
     }
 
+    SECTION("Cloning")
+    {
+        CHECK_ATTR_COUNT( 0 );
+
+        m_grid->GetOrCreateCellAttrPtr(0, 0)
+            ->SetClientObject(new wxStringClientData("test"));
+        CHECK_ATTR_COUNT( 1 );
+
+        m_grid->SetAttr(0, 1, m_grid->GetOrCreateCellAttrPtr(0, 0)->Clone());
+        CHECK_ATTR_COUNT( 2 );
+
+        wxClientData* const
+            data = m_grid->GetOrCreateCellAttrPtr(0, 1)->GetClientObject();
+        CHECK( static_cast<wxStringClientData*>(data)->GetData() == "test" );
+    }
 
     // Fill the grid with attributes for next sections.