Push/pop visible scrollback by VT-command

[o-sdn-o]

We need to come up with a new VT sequence to temporarily save the visible scrollback lines (something like Push/PopViewport). This will allow Far Manager to get rid of the use of the ReadConsoleOutput* API. This should work for both the normal buffer and the alternate one.

The use case looks like the following:

• At startup, FM outputs two '\n' to reserve space for its prompt and keyboard shortcuts.
• FM makes the normal buffer's copy by the PushViewport.
• FM draws its interface on top of the scrollback, while not allowing any scrolling of the buffer.
• Before each launch of an external program, FM emits PopViewport and executes the program.
• After the program terminates, FM reserves two lines for its prompt and keyboard shortcuts (2x'\n') and emits PushViewport.
• FM draws its UI on top of the scrollback.
• When FM disables/enables its panels, it emits PopViewport followed by the PushViewport command and draws its panels there on top of the original scrollback.
• At exit, FM restores the scrollback just by emitting PopViewport.

push: CSI 411 s
pop:  CSI 411 u

411 means All.