Fix regression in wxGTK wxFilePickerCtrl due to wxFileDialog changes.

Revert the changes of r72979 and fix wxFileDialog::GetDirectory() return
value bug (see #14786) differently to avoid breaking wxFilePickerCtrl which
relies on getting wxEVT_BUTTON event when the associated dialog is dismissed.

Closes #15218.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74043 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775