allow to show the statusline on top

This is an experimental feature that allows to display the statusline in
the tabline. It might still be a bit rough, but seems to work so far.

Remaining problem:
- Mode changes are not immediately detected, only after moving the
  cursor

fixes #1388
closes #1867

autoload/airline/extensions/tabline.vim

@@ -33,6 +33,11 @@ function! s:toggle_off()
 endfunction
 
 function! s:toggle_on()
+  if get(g:, 'airline_statusline_ontop', 0)
+    call airline#extensions#tabline#enable()
+    let &tabline='%!airline#statusline('.winnr().')'
+    return
+  endif
   call airline#extensions#tabline#autoshow#on()
   call airline#extensions#tabline#tabs#on()
   call airline#extensions#tabline#buffers#on()
@@ -58,12 +63,25 @@ function! s:update_tabline()
     return
   endif
   call airline#util#doautocmd('BufMRUChange')
+  call airline#extensions#tabline#redraw()
+endfunction
+
+function! airline#extensions#tabline#redraw()
   " sometimes, the tabline is not correctly updated see #1580
   " so force redraw here
   if exists(":redrawtabline") == 2
     redrawtabline
   else
-    let &tabline = &tabline
+  " Have to set a property equal to itself to get airline to re-eval.
+  " Setting `let &tabline=&tabline` destroys the cursor position so we
+  " need something less invasive.
+    let &ro = &ro
+  endif
+endfunction
+
+function! airline#extensions#tabline#enable()
+  if &lines > 3
+    set showtabline=2
   endif
 endfunction