frontend: allow button input widgets to control data-lake variables

Current behaviour

A button input widget can only trigger an Action, which makes it unsuitable for momentary control situations.

Expected or desired behaviour

Allow buttons to either trigger an Action, or momentarily set (mousedown) and clear (mouseup and mouseout, after a mousedown) a boolean data-lake variable.

This would match the existing support for joystick button mapping, and moves us closer to being able to nicely provide on-screen joystick support.

Additional context

Relevant to this forum request, for momentary relay control.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.