Fix wxGTK's wxFileDialog:Get/SetFilename() to be in sync.

Fix wxGTK's wxFileDialog:Get/SetFilename() to be in sync.

The m_fileName and m_dir variables, managed by the base class, were out
of sync when the dialog wasn't shown yet, because the wxGTK
implementation didn't always set them. GetFilename() following
SetFilename(x) could return something other than x. Fixed by calling the
base class version.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72948 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775