add function to save all output directly

[Karl-Philipp Richter]

Currently it's only possible to save output by selecting all output and copying it into an editor. It there's a lot of output (> 1M lines) the selection process can take up to several minutes in which `gnome-terminal` either crashes are enqueues mouse clicks to be processed after the selection finished which means that the click immediately undoes the selection. In order to overcome the latter it'd be nice to be able to save to a file directly.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: gnome-terminal 3.6.2-0ubuntu1
Uname: Linux 3.19.0-031900-generic x86_64
ApportVersion: 2.14.7-0ubuntu8.1
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Feb 13 09:01:05 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-01-26 (17 days ago)
InstallationMedia: Ubuntu 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
SourcePackage: gnome-terminal
UpgradeStatus: No upgrade log present (probably fresh install)