Save and restore file history in docview sample.

Save and restore file history in docview sample.

Show how the file history can be saved to and restored from wxConfig.

This makes it much easier to test file history related stuff as the history
doesn't need to be recreated during every sample run.

It is also closer to what real applications using docview framework do as the
file history is relatively useless if it's not saved.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68332 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775