Add 'G' keybinding to folder and search view that triggers external poll

Add 'G' keybinding to folder and search view that triggers external poll

The new functions first check if an external poll script has been defined in
the variable 'notmuch-external-refresh-script and if yes, runs that script
before executing the existing refresh function (which is bound to '=')

This can be used to have 'G' mimic the mutt behavior of polling an external
mail server - or if the mail polling is already automatic, it can trigger
the call to notmuch new and any necessary automatic tagging of new email.

Signed-off-by: Dirk Hohndel <hohndel@infradead.org>