Extract wxModalDialogHook from wx/testing.h into its own wx/modalhook.h,
extend it to allow to be notified not only about showing modal dialogs but
also about dismissing them and document it and show its use in the dialogs
sample.
Also replace all the WX_TESTING_SHOW_MODAL_HOOK macros occurrences with
WX_HOOK_MODAL_DIALOG.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74037 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
