tabline: prevent flicker on Windows

calling settabvar() while evaluating the 'tabline' setting apparently
causes flicker on Windows. Fall back to using `:let t:var` to store the
content in the current tabpage.

This is not as good as using `settabvar()` since we cannot store the
title for other tabs, but at least it should prevent the flicker and at
the same time at least cache the title for the current tabpage.

autoload/airline/extensions/tabline.vim

@@ -147,7 +147,12 @@ function! airline#extensions#tabline#title(n)
   endif
 
   if exists("*settabvar") && !empty(title)
-    call settabvar(a:n, 'title', title)
+    " don't use settabvar, it causes a refresh,
+    " which in turn causes flicker on windows
+    "call settabvar(a:n, 'title', title)
+    if tabpagenr() == a:n
+      let t:title = title
+    endif
   endif
   return title
 endfunction