This reverts commit 29c78ce841 and all other fixes
that have been made afterwards.
The motivation is that use-package is seen by many as a replacement for
`require`. Is use-package always defer the loading of packages then is breaks
this use case, this does not respect POLA so even if it was making Spacemacs
loading faster (up to 3s faster on some startup on my machine) we just cannot
use it, it would be irresponsible. Spacemacs should be easy to use, loading
performance will come with time but it is not a priority.
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.
