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/quickfix.vim

@@ -0,0 +1,25 @@
+let s:suite = themis#suite('quickfix')
+let s:assert = themis#helper('assert')
+
+function! s:suite.before_each()
+  let g:lightline = {}
+  call lightline#init()
+  tabnew
+  tabonly
+endfunction
+
+function! s:suite.quickfix_statusline()
+  call setloclist(winnr(), [])
+  lopen
+  wincmd p
+  call setloclist(winnr(), [])
+  for n in range(1, winnr('$'))
+    let statusline = getwinvar(n, '&statusline')
+    call s:assert.match(statusline, 'lightline')
+    if has('patch-8.1.1715')
+      call s:assert.match(statusline, n == 1 ? '_active_' : '_inactive_')
+    else
+      call s:assert.match(statusline, n != 1 ? '_active_' : '_inactive_')
+    endif
+  endfor
+endfunction