Use mouse position to find the item for selection events in wxMSW listbox.

Use mouse position to find the item for selection events in wxMSW listbox.

Using LB_GETCARETINDEX doesn't work when the mouse is used to make selection
because it always returns the index of the last item, even if the mouse is
clicked below it, on an area without any listbox items. So use the mouse
position to find the item in this case but still use LB_GETCARETINDEX to find
the item when the keyboard is used.

This required adding a flag to wxListBox storing the kind of the last input
message that it received as there doesn't seem to be any way to determine how
the message was generated otherwise.

This code will be refactored/improved further in the next two commits.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64498 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775