Add lambda-friendly wxDialog::ShowWindowModalThenDo().

Add lambda-friendly wxDialog::ShowWindowModalThenDo().

Add a convenience ShowWindowModalThenDo() variant of ShowWindowModal()
that takes a functor argument and calls it when the dialog is closed.

This is, of course, particularly useful when the argument is a C++11
lambda, especially when having more than one window-modal dialog invoked
from the same window, which can get messy quickly with all the
wxEVT_WINDOW_MODAL_DIALOG_CLOSED handlers.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74775 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775