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/init.vim

@@ -194,7 +194,7 @@ function! airline#init#sections()
     let g:airline_section_y = airline#section#create_right(['ffenc'])
   endif
   if !exists('g:airline_section_z')
-    if winwidth(0) > 80
+    if airline#util#winwidth() > 80
       let g:airline_section_z = airline#section#create(['windowswap', 'obsession', '%3p%%'.spc, 'linenr', 'maxlinenr', spc.':%3v'])
     else
       let g:airline_section_z = airline#section#create(['%3p%%'.spc, 'linenr',  ':%3v'])