centralized Esc key handling for closing the dialogs in wxDialogBase:

centralized Esc key handling for closing the dialogs in wxDialogBase:
1. added wxDialogBase::OnCharHook() and removed this event handler from
   all the other ports
2. also removed ad hoc code doing the same thing in wxMSW (MSWProcessMessage()
   override in wxDialog) and wxGTK (in gtk_window_key_press_callback())
3. reimplemented EmulateButtonClickIfPresent() portably and also moved it
   to wxDialogBase from wxMSW wxDialog

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40686 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775