"Add a variable" action is in the wrong category

In D7 this was in the "Data" category.

In D8, it was initially put in "Data", but then was moved to "Variable", which was a newly-defined category which doesn't have any other actions or conditions or events.

This change was made by commit 784fe5cc which doesn't have an associated issue in the issue queue, so there was no discussion of this change and no reason presented for the change.

IMO categories are useful only if they contain a collection of related tasks. They serve to organize the available actions and improve UX because actions may be found more easily. Adding a variable is clearly related to all the other actions in the "Data" category (setting a variable value, etc.). Adding a variable was in the "Data" category in D7, so that's where Rules users expect to find it. The Rules documentation and examples are mostly D7 based, so when people see there's an action to add a variable but can't find it in the expected place in D8, they assume it doesn't exist in D8. A "Variables" category which has only one entry doesn't serve to organize anything, and it is not the expected place to find this action. Additionally, "Variables" is all the way at the bottom of the list, so it's unlikely you will see this while casually scrolling if it didn't show up where you expected.

This patch changes the category back to "Data". If you want to test this, be sure to clear your caches after applying the patch.