Accessibility: Add support for performing actions on elements

[tronical]

With accessibility APIs there are ways that assistive tech like screen readers can activate elements "programmatically". This is sometimes emulated by the system using a synthetic mouse click (VoiceOver on macOS seems to like doing that), but for example the AccessKit APIs have dedicated actions that can be run, such as "Increment", "Decrement", "Expand", "Collapse", "ShowContextMenu", "SetValue" (slightly more generic), or even "Default" (presumably press on a Button, for example).

We should allow defining such actions in .slint and invoke them from the run-time library if the accessibility APIs request for their invocation.

Perhaps we can use implicit callbacks for this?

accessibility-action-expand => { my code here to expand }
accessibility-action-increment => { self.increment(); }

If such a callback is defined on an element, then we can report to the OS that this action is supported and invoke correspondingly.

[ogoffart]

Another possibility:

accessibility-action(action, value /*: string*/) => {
   if (action == AccessibilityAction.expand) {
       // ... 
       return accept;
   } else if (action == AccessibilityAction.increment) {
      // increment
      return accept;
   } else if (action == AccessibilityAction.set-value) {
      root.value = value;
      return accept;
   } else { 
       return reject
   }
}

But this seems more verbose than many accessibility-action-* callbacks and also better types. (and in the runtime, the compiler can anyway lower both solution to the same code, so runtime consideration doesn't matter here, it's mostly what would give the better API)

[ogoffart]

I had some thought about it, here is what i think we need to do:

Runtime:

• [ ] Add an (internal) enum AccessibleAction{ Default, Expand, SetValue(f64), SetText(SharedString), ...}
• [ ] Add ad function in the ItemTreeVTable such as fn accessible_action(Pin<VRef<ItemTreeVTable>>, item_index: u32, ation: AccessibleAction)
• [ ] We also need an accessible_supported_actions function in the vtable that returns the set of supported action for an item. (as a bitflags!?)
• [ ] In the winit and Qt backend, implement code that calls this accessible_action when coming from the accessible interface

Compiler:

• [ ] Similar to the accessible- property, add accessible-action- callbakcs. They should take a parameter so that accessible-action-set-text would take a string.
• [ ] Generate the accessible_action implementation in the ItemTree implementation, that calls the code for the given callback.
• [ ] And generate the code for accessible_supported_actions based on what callback are being re-implemented

Widgets:

• [ ] add the accessible-action* callbacks to all relevant widgets
• [ ] Including a test in tests/cases/widgets that it works. (need https://github.com/slint-ui/slint/pull/4583)

[ogoffart]

This was done in 1.6