Don't send listbox selection events when mouse wasn't clicked on an item.

Don't send listbox selection events when mouse wasn't clicked on an item.

Windows sends events even if the mouse is clicked in the area below the items,
filter them out by checking the item under the mouse ourselves.

Also use synchronous GetMessagePos() function to get the mouse position
instead of the asynchronous GetCursorPos() which was wrongly used by the old
code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64499 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775