Fix a bug about termguicolors option

We can not detect whether nvim-qt is loaded when sourcing `init.vim`.
If termguicolors option is not set, we use a better color (badwolf in this case).
Gruvbox8 is terrible when termguicolors is not set.

ui.vim

@@ -2,15 +2,13 @@
 "{{ General settings about colors
 " Enable true colors support (Do not use this option if your terminal does not
 " support true colors! For a comprehensive list of terminals supporting true
-" colors, see https://github.com/termstandard/colors and https://bit.ly/2InF97t.
-if exists("&termguicolors")
-    if $TERM == "xterm-256color" || exists('g:GuiLoaded')
+" colors, see https://github.com/termstandard/colors and
+" https://bit.ly/2InF97t)
+if $TERM == "xterm-256color"
     set termguicolors
 else
     set notermguicolors
 endif
-endif
-
 " Use dark background
 set background=dark
 "}}
@@ -19,6 +17,7 @@ set background=dark
 """"""""""""""""""""""""""""gruvbox settings"""""""""""""""""""""""""""
 " We should check if theme exists before using it, otherwise you will get
 " error message when starting Nvim
+if &termguicolors
     if utils#HasColorscheme('gruvbox8')
         " Italic options should be put before colorscheme setting,
         " see https://goo.gl/8nXhcp
@@ -30,6 +29,16 @@ if utils#HasColorscheme('gruvbox8')
     else
         colorscheme desert
     endif
+else
+    if utils#HasColorscheme('badwolf')
+        let g:badwolf_darkgutter = 0
+        " Make the tab line lighter than the background.
+        let g:badwolf_tabline = 2
+        colorscheme badwolf
+    else
+        colorscheme desert
+    endif
+endif
 
 """"""""""""""""""""""""""" deus settings"""""""""""""""""""""""""""""""""
 " colorscheme deus
@@ -49,11 +58,5 @@ endif
 " " theme_style can be 'default', 'dark' or 'palenight'
 " let g:material_theme_style = 'dark'
 " colorscheme material
-
-"""""""""""""""""""""""""""badwolf settings""""""""""""""""""""""""""
-" let g:badwolf_darkgutter = 0
-" " Make the tab line lighter than the background.
-" let g:badwolf_tabline = 2
-" colorscheme badwolf
 "}}
 "}