Introduce disabling switching to alternative screen

One thing that has kept me from using any libvte-based terminal emulators was the inability to turn of switching to the alternate screen using the decset 1047/1049 codes. Programs like vi and less use them when they start/terminate, which means whenever i look at a manual page and quit to the shell, the manpage is gone. Likewise, i edit something, quit vi, and whatever i edited is gone from the screen.

I know i can change this behaviour by changing the termcap entry - which is a hassle when you ssh around a lot - or i can change TERM to something that doesn't set ti and te - which is likely to break other stuff as well.

In XTerm, this behaviour can be controlled using the titeInhibit resource, or the middle mouse button menu. Other programs, like putty, allow to change this as well, and googling for "disable alternative screen" suggests there's a good amount of people who dislike this "feature". So, to be able to add this option to gnome-terminal, xfce-terminal, and various others, libvte needs to provide this ability, which this pull requests adds.