Disable group merge for single item or non-group items

[HiroyasuNishiyama]

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

Proposed changes

Groups > Merge selection menu do not merge group for single element or non-group items.
The operation for these items is the same as for Groups > Group selection.
Therefore, This PR try to disable the Group selection menu for these cases.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/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 decreased (-0.02%) to 77.31% when pulling a15adc4 on node-red-hitachi:fix-group-merge-activation into 81a4f42 on node-red:dev.

[knolleary]

Thanks for this - a good change to make.

Can I make one suggestion - I notice this function now calls selection.nodes.filter three separate times for the different menu options.

How about changing it to call selection.nodes.forEach just once at the start of the function to check for two different conditions used later on - one for if there is a group in the selection and one if nothing in the selection is already in a group.

It's a small improvement but I'm keen to improve the efficiency of the editor code - especially on paths that could get called alot.

[HiroyasuNishiyama]

Hi. Committed a fix to share codes.