Set wxTextAreaBase::m_filename in the methods of this class itself.

Set wxTextAreaBase::m_filename in the methods of this class itself.

m_filename member was declared in wxTextAreaBase but set only in the
overridden wxTextCtrlBase methods. This meant that it wasn't updated correctly
in wxStyledTextCtrl which also derives from wxTextAreaBase and so saving
SaveFile() with empty file name didn't work there even when the control had
been originally loaded from a file.

Move the code updating m_filename to wxTextAreaBase itself to fix this. This
also simplifies the code as it's now not necessary to override
Do{Load,Save}File() in wxTextCtrlBase at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71026 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775