This change adds a new pair of methods to ITermDispatch:
PushGraphicsRendition and PopGraphicsRendition, and then plumbs the
change through AdaptDispatch, TerminalDispatch, ITerminalApi and
TerminalApi.

The stack logic is encapsulated in the SgrStack class, to allow it to be
reused between the two APIs (AdaptDispatch and TerminalDispatch).

Like xterm, only ten levels of nesting are supported.

The stack is implemented as a "ring stack": if you push when the stack
is full, the bottom of the stack will be dropped to make room.