Naive impl of "selected" function in expression editor
Refactor: generic expression parse/construct
Set "selected" operator on drop multi-select
Fail to advanced editor on wrong number of args

This could happen if the user typed an expression like `selected(/data/things, 'thing1', 'thing2')` (which incidentally may not be a valid function call). Previously the third argument would have been silently discarded.
Do not use "selected" function on single-select

The Select mug extends the MSelect, and it inherited its defaultOperator. This fix overrides that default.
Merge pull request #189 from dimagi/104232-selected-function

Add "has selected value" operation to simple expression builder