Use libXtst again with CMake (#1908)

* Use libXtst again with CMake

It seems that Xtst was never used after the transition to CMake.

This is related to https://github.com/stepmania/stepmania/issues/1902.

* Travis: install libxtst-dev

StepmaniaCore.cmake

@@ -395,6 +395,12 @@ elseif(LINUX)
     set(HAS_XRANDR TRUE)
   endif()
 
+  set(HAS_LIBXTST FALSE)
+  if(WITH_LIBXTST)
+    find_package(Xtst REQUIRED)
+    set(HAS_LIBXTST TRUE)
+  endif()
+
   set(HAS_XINERAMA FALSE)
   if(WITH_XINERAMA)
     find_package(Xinerama REQUIRED)