We called g_signal_connect("selection_get") in wxClipboard code each time its
AddData() method was called. This resulted in progressive but noticeable
slowdown as the handler was called more and more times.
Only connect to the handler once now.
Closes #15038.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73518 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775