Update each window only once in s:on_window_changed.
Update each window only once in s:on_window_changed.

We might come there several times for different autocommands.

The key uses bufnr/winnr and the number of total windows, and is stored
for the tab.

dalguji/vim-airline,tschundler/dotfiles,HarmtH/vim-airline,tschundler/dotfiles,bling/vim-airline,chrisbra/vim-airline,rchicoli/vim-airline,tschundler/dotfiles,vim-IDE/vim-airline,yzhou61/vim-airline,unphased/vim-airline