Move member m_ variables to be protected rather than public to force usage of Set...

Move member m_ variables to be protected rather than public to force usage of Set/Get rather than allow direct access.  Compatibility with 2.4 keeps members private, and also creates a class to allow continued usage of m_commandString. GetString() will now get the text control's string when called, rather than store it and return it every time (huge performance gain by doing this, esp. on GTK2)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31455 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775