Use native hint wxTextCtrl support in wxSearchCtrl.

Use native hint wxTextCtrl support in wxSearchCtrl.

Instead of using broken emulation of the support for hints available in the
text control, use the real wxTextCtrl::SetHint() for SetDescriptiveText()
implementation in the search control.

This looks better and, more importantly, fixes the bug when searching for the
string equal to the current descriptive text searched for an empty string
instead.

Closes #13324.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68237 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775