Refactor wxStyledTextCtrl to share common file save/load code.

Refactor wxStyledTextCtrl to share common file save/load code.

Keep the code for saving and loading text contents from files in a single
place instead of doing it differently in wxTextCtrl and wxStyledTextCtrl.

This required adding Set/GetValue() methods to wxTextAreaBase just so that its
DoLoad/SaveFile() could use them, even if they are the same as wxTextEntryBase
methods and are overridden in wxTextCtrlBase to be implemented in terms of the
latter.

Notice that wxRichTextCtrl might need to be refactored to use this code too in
the future.

Also notice that this reverts the change of r62081 which replaced SetValue()
with ChangeValue() in DoLoadFile() as wxTextAreaBase only has SetValue() and
it's not worth adding ChangeValue() to it too just to preserve this recent
change in behaviour.

Closes #10715.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62139 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775