change events for updating the statusline, from BufWinEnter,FileType to BufEnter (close #352)

- fix the active statusline after updating the quickfix/location list
- use g:qf_disable_statusline to disable the statusline of the default ftplugin
- for before Vim 8.1.1715, keep using FileType event to overwrite the statusline of the quickfix window

test/.themisrc

@@ -18,3 +18,5 @@ endfunction
 function! SID(name) abort
   return function(printf("\<SNR>%d_%s", s:sid('autoload/lightline.vim'), a:name))
 endfunction
+
+filetype plugin on