Implement the `XTPUSHSGR` and `XTPOPSGR` control sequences (see #1796).
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.
Partial pushes (see the description of `XTPUSHSGR` in Issue #1796) are
implemented per xterm spec.
## Validation Steps Performed
Tests added, plus manual verification of the feature.
Closes#1796
72cbe59078