Decouple multiple element type selection from RelatedToConditionRule

[rynpsc]

The "Related To" condition rule allows the user to choose from any element type via RelatedToConditionRule which inherits from BaseElementSelectConditionRule.

Could a multiple property be added to BaseElementSelectConditionRule and then some of the logic within RelatedToConditionRule be moved to BaseElementSelectConditionRule instead? This would allow easier reuse of the multiple element selection for other purposes other than for relations.

For example in Craft Commerce the "Matching Items" for a discount could be moved to use the condition builder allowing things other than categories to be selected, or in a plugin which collects "comments" for any Element, comments could be filtered on the element index based on element.

This would also be relevant to craftcms/commerce#2796

I'm happy to make a PR.

[brandonkelly]

This would be a breaking change at this point, as it would involve renaming BaseElementSelectConditionRule::getElementId() / setElementId(), or at least changing their signatures. Will add it to the 5.0 list though.