Optimize allocating default cells in wxPGProperty::EnsureCells

Use embedded 'resize' function to alloc new slots at once instead of doing this in an iterative way.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77506 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775