SELECTEDVALUE option in automatic aggregations

Add a new "Selected Value" option to the field wells and default aggregations that reproduces the behavior of a SELECTEDVALUE measure in DAX. (Returns the value when the column, as filtered, has one distinct value. Otherwise, the field is empty.)

This would be extremely powerful when data contains numerous categorical/descriptive values or where a lot of interactivity is intended. (For example, this would make it easy for table(s), card(s), etc. to function like a tooltip displaying details only when a value is selected. First, Last, Minimum, Maximum, or Do Not Summarize are not precise enough and could lead to mistaken readings. And creating multiple individual SELECTEDVALUE() measures for each column in question is extremely tedious and unneccessarily clutters the data model if they are to be used only once.