In a multi-window scenario, it is possible to return focus to the last
accessed window via n_CTRL-W_p. However, in the case of a multi-tab
scenario, there was previously no way to return focus to the last
accessed *tab*. Here, that ability is added via n_g<tab>.
Additionally, the index of the previous tab is exposed via
tabpagenr('#'), mirroring the existing functionality of winnr('#').