Invalidate wxListBox best size immediately without waiting for idle time.

Invalidate wxListBox best size immediately without waiting for idle time.

Since r53743 the listbox best size was only invalidated during idle time but
this meant that it could be laid out using incorrect old best size. So while
we still defer (expensive) horizontal extent calculation until later, do
invalidate the best size immediately to ensure the listbox is laid out
correctly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64246 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775