[GPU-Plugin] Major refactor 2 (#2664)

* Change cmake option

* Move source files

* Move google tests

* Move python tests

* Move benchmarks

* Move documentation

* Remove makefile support

* Fix test run

* Move GPU tests

doc/_static/xgboost-theme/navbar.html

@@ -23,6 +23,7 @@
             <li><a href="{{url_root}}jvm/index.html">JVM</a></li>
             <li><a href="{{url_root}}julia/index.html">Julia</a></li>
             <li><a href="{{url_root}}cli/index.html">CLI</a></li>
+            <li><a href="{{url_root}}gpu/index.html">GPU</a></li>
           </ul>
         </li>
         {% endfor %}