Fit modeless preferences dialog to its contents too.

When a preferences dialog is shown non-modally, it should still fit to the
contents of its pages, so move the code added for the modal case in r73624
to the base wxGenericPreferencesEditorImplBase class.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74040 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775