Use correct style for the dialog shown by generic file/dir pickers.

Use correct style for the dialog shown by generic file/dir pickers.

The picker styles don't make sense for the button so we don't use them when
creating the button but we do need to somehow use the style the picker was
created with to create an appropriate dialog when it's clicked.

Fix the problem by simply storing the style in a member variable and using it
instead of wxWindow::m_windowStyle.

Closes #11635.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63654 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775