Extract common parts of wxData{In,Out}putStream in a common base class.

Extract common parts of wxData{In,Out}putStream in a common base class.

No real changes, just put BigEndianOrdered() and SetConv() methods and the
corresponding fields in a common wxDataStreamBase class instead of duplicating
them in wxDataInputStream and wxDataOutputStream.

This will make it simpler to add more features common to both classes in the
future, see #10625.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73933 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775