Custom action justification

[scaredcat]

Currently, things are a little magic in that the first custom action specified will always be right-justified, and all remaining ones are left-justified. If the user doesn't want any actions on the right, they can set the first custom action in the array as null.

This should be changed such that a justification prop or something can be specified within each defined custom action, which will add it to either the left or right groups.