Move uninstall target to GLFW3 folder

Fixes #1129.

CMakeLists.txt

@@ -401,6 +401,7 @@ if (GLFW_INSTALL)
         add_custom_target(uninstall
                           "${CMAKE_COMMAND}" -P
                           "${GLFW_BINARY_DIR}/cmake_uninstall.cmake")
+        set_target_properties(uninstall PROPERTIES FOLDER "GLFW3")
     endif()
 endif()