Add 'Redo (un-undo)' function

[k-toumura]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

This PR adds a 'redo' function to the Node-RED flow editor.
By this function, developers can cancel an undo operation.

Keyboard shortcut of redo is assigned to 'Ctrl-Shift-Z'.

To discuss implementation details, I opened this PR as a draft at first.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the mailing list/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 77.535% when pulling b749a27 on node-red-hitachi:dev-redo into a376d6e on node-red:dev.

[k-toumura]

Fixed minor bug and tested handling of each history event, so I publish this PR for review.

[k-toumura]

Keyboard shortcut of redo is assigned to 'Ctrl-Shift-Z'.

I assigned this shortcut based on Apple's Human Interface Guideline, and Ubuntu's manual.
On the other hand, Microsoft's guideline suggests to use 'Ctrl-Y' for redo function.

Which keyboard shortcut is better for redo function?

We could assign both of shortcuts for redo (add each definitions in keymap.json), but this is somewhat confusing.

[k-toumura]

Which keyboard shortcut is better for redo function?

In Chrome and Safari on macOS, ctrl-y is already assigned to 'show all history'.
This assignment can be overridden in workspace, but it can't in node editor tray (It is not clear why it can't...).

[knolleary]

After you opened this, an issue was spotted and fixed in the code to undo 'creating subflow from selection'. The same fix was needed into the inverse event you had introduced - which I have applied here: c8acc6a

Thanks!