There is a space in the buffer name of undo-tree. The popwin did not display right and the function `undo-tree-visualizer-quit` also seems not work well until I correct the buffer name.
Create new layers:
- spacemacs-navigation: contains packages whose principal goal is navigation
- spacemacs-modeline: contains packages about mode line
Merge spacemacs-ui and spacemacs-ui-visual into layer spacemacs-visual.
