emulation: Add support for DEC 1007 to set the alternate scroll mode

By default, the mouse wheel sends cursor up/down keycodes in the
alternate screen. This adds an escape sequence (DEC 1007) that allows
turning this off (and on again).

For compatibility with ubuntu's ******** patched vte, also add a
(deprecated, skip) public API that has the expected name but does nothing.

Based on patches from ubuntu, and Egmont Koblinger.

https://bugzilla.gnome.org/show_bug.cgi?id=518405
https://bugzilla.gnome.org/show_bug.cgi?id=709060