Only give capture to wxPopupTransientWindow child if it's the only one.

Only give capture to wxPopupTransientWindow child if it's the only one.

This removes the arbitrariness of using the first child which didn't make any
sense when the popup had several controls inside it, e.g. in wxRichToolTip
case this resulted in the mouse capture being given to wxStaticBitmap showing
the tooltip icon even though it didn't need it at all.

This probably doesn't really change the observable behaviour but seems more
logical.

See #15288.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74680 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775