Use wxNavigationEnabled<> for keyboard navigation in generic wxSearchCtrl.

Use wxNavigationEnabled<> for keyboard navigation in generic wxSearchCtrl.

Derive generic wxSearchCtrl implementation from wxNavigationEnabled<> to
ensure that TAB navigation works correctly in it. While it did work before for
search controls without wxTE_PROCESS_ENTER style (because this wasn't handled
by this control itself at all in fact), it stopped working as soon as this
style was used in wxMSW because then the navigation was implemented by
manually calling wxWindow::Navigate() and this requires wxControlContainer
support.

Use wxNavigationEnabled<> to easily add it to wxSearchCtrl.

See #12808.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68364 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775