do not cache status of statusline per tabpage
do not cache status of statusline per tabpage

When cacheing the values of buffer number, window number per tabpage, we
might forget to update the statusline when it is actually needed, e.g.
on `:tab help h | norm! gt` which would then still display "Help" also
we are not in a help window anymore.

Therefore cache those values (including current tabpage number)
globally.

fixes #1253

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