separate out vimtabs fragment as a public function

Author: zefei

autoload/wintabs/ui.vim

@@ -1,6 +1,6 @@
 " generate tabline, very straight forward
 function! wintabs#ui#get_tabline()
-  let spaceline = s:get_spaceline()
+  let spaceline = [g:wintabs_renderers.line_sep(), s:get_spaceline()]
   let bufline = s:truncate_line(
         \0,
         \s:get_bufline(0),
@@ -43,6 +43,11 @@ function! wintabs#ui#reset_statusline(window)
   return ''
 endfunction
 
+" public function to expose fragment for vimtabs
+function! wintabs#ui#get_vimtabs_fragment()
+  return wintabs#element#render(s:get_spaceline())
+endfunction
+
 " private functions below
 
 " generate bufline per window
@@ -239,7 +244,7 @@ function! s:get_spaceline()
     return ['', 0]
   endif
 
-  let line = [g:wintabs_renderers.line_sep()]
+  let line = []
   let active_index = tabpagenr()
 
   for tab in range(1, spaces)

doc/wintabs.txt

@@ -211,6 +211,8 @@ default: value of |statusline| option
                 content to tabline.
                 By default, this option is set to what 'original statusline' 
                 is. If vim-airline is installed, airline#statusline() is used.
+                Set this option to a custom function like "%!MyStatusline()" 
+                if you want to show complex and customized status on tabline.
                 Set this option to empty string to disable this behavior.
 
 *g:wintabs_ignored_filetypes*