Add align actions to editor

[knolleary]

• New feature (non-breaking change which adds functionality)

Proposed changes

Adds align actions to the editor:

• core:align-selection-to-left - alt-a l
• core:align-selection-to-center - alt-a c
• core:align-selection-to-right - alt-a r
• core:align-selection-to-top - alt-a t
• core:align-selection-to-middle - alt-a m
• core:align-selection-to-bottom - alt-a b
• core:distribute-selection-horizontally - alt-a h
• core:distribute-selection-vertically - alt-a v

ToDo:

• Consider if align-selection-to-group should apply to nodes inside groups - it doesn't currently
• Add actions to distribute horizontally/vertically
• Add align menu to main menu

https://trello.com/c/iYmbLYT1

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 67.637% when pulling 192b542 on view-tools into f1e7ec0 on dev.

[knolleary]

Final update - have added menu options to for these actions, along with some other common actions.

Also updated the menu to display keyboard shortcuts against menu items if one is set.