frontend: joystick: expose linked Actions for data-lake variables

Current behaviour

Data-lake variables are presented by name in the joystick button function configuration window, but for that to be useful requires the variable name to represent all functions it represents. That is generally ok for single-use variables, but becomes challenging if the variable is used in multiple functions, like would be useful for #1880.

Expected or desired behaviour

1. In the button function configuration interface, list data-lake variables together with any Actions that make use of and/or are triggered by them
2. In the joystick configuration interface, show button functions that have been linked to data-lake variables as/with the Actions that use and/or are triggered by those variables
   • If an Action has a single variable as its only trigger, it could be displayed as a primary function, with any other linked Actions displaying as smaller/secondary
   • Ideally it would be possible for the Action listings to jump to (or at least preview) the definition of the corresponding Action

Finding linked Actions may require a multi-level search, to account for compound variables. Using only direct links (i.e. Actions that have the variable in question as one of their registered triggers) could substantially simplify the logic, but would mean Actions with time-based triggers wouldn't get displayed (which could be relevant for #1759, for example).

Prerequisites

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

Start with the button function configuration and joystick configuration interfaces, then trace how data-lake variables and Actions are represented and linked. Done means both interfaces expose the relevant linked Actions, including the intended handling of direct or compound links and navigation or preview of Action definitions.