Use "Cmd" instead of "Ctrl" in code for Mac-specific keyboard shortcuts

[njx]

Currently, we use "Ctrl" even when declaring keyboard shortcuts that are Mac-specific (which automatically maps to Cmd at runtime). This seems confusing--it seems clearer to specify Cmd directly in these cases (especially because the Mac also has a "control" key).

[peterflynn]

Should be easy since it looks like KeyBindingManager already checks for both "Cmd" and "Ctrl"... albeit currently it prints a spurious warning message if you use "Cmd".

[peterflynn]

See also related issue #938 about making the Mac Ctrl key usable for keyboard shortcuts. There are several suggestions there for key modifier representation that might be relevant to this discussion.

[tvoliter]

Nominating for sprint 10. Since our menu and keyboard API is a big part of our extensibility story we should get this right before lots of extensions are written.

[jasonsanjose]

FBNC @njx

[jasonsanjose]

No commands changed, so there's nothing to verify in the app. Can't test Ctrl+Tab for document switching because of #938.

[tvoliter]

NJ has lots of meetings this afternoon, so I'll take care of closing this one.

[tvoliter]

Verified. Closing.