frontend: allow Input widgets to trigger Actions

[ES-Alexander]

Current behaviour

Most Input widgets can only set the value of a variable, which means actually triggering an Action that uses that variable requires doing so by some other means, which can be helpful sometimes, but can also be an awkward extra step, that may clutter up the UI with redundant elements.

Expected or desired behaviour

1. Somewhat building off Actions: support release event callbacks #1475, widgets that allow changing a variable could also have an option to trigger an Action when that variable is changed
2. For continuous input elements it may be worth providing a frequency limit on how often the trigger occurs (e.g. while moving a slider or dial), or setting it to a flag value like 0 or -1 to only trigger when the element is released

Two alternative/additional approaches would be to:

3. provide a separate area where Actions can be set to trigger at a set frequency, and maybe subject to some user-defined conditions
   • This is very versatile, but may be unintuitive to some users
4. provide a separate area where data-lake variable listeners can be set up, to trigger Actions when the variables change
   • This seems like it could have poor performance

but the separation of those from where the variable changers are configured may make them less intuitive than just single Action triggers where the variables are configured 🤷

Prerequisites

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