Clear the search control automatically when it's "Cancel" button is pressed.

Clear the search control automatically when it's "Cancel" button is pressed.

This should be the desired behaviour in the vast majority of cases, so do it
by default.

Replace the useless OnSearchButton() doing nothing with search button events
with OnCancelButton() handling cancel button events and clearing the control.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72581 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775