frontend: provide a consolidated page of Action triggers

[ES-Alexander]

Current behaviour

Joystick buttons and Input Button widgets can both trigger Actions, Actions can potentially call other Actions, and #1538 may add more widget-based triggers. That's starting to get quite fragmented, so it could become tricky to determine where a given Action is being triggered from.

Expected or desired behaviour

• Action calls (and responses) should be included in the Cockpit system logs (if they aren't already)
  • Ideally this should include some reference to what source called them (e.g. a widget or Action name)
• An extra tab in Settings / Actions that specifies all known Action triggers would be handy
  • Ideally this would include some basic parsing of custom JS Actions to see whether they call other Actions

Prerequisites

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