Fix assert when changing the filter selection on a multiple selection wxGenericDirCtrl.

Fix assert when changing the filter selection on a multiple selection wxGenericDirCtrl.

Because ExpandPaths selects the item we can simply grab a list of selected paths and then expand them all in the multiple selection case. The single selection case is unchanged.

Closes #12340

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65591 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775