Consider Simpler Event Types

[TTWNO]

My Odilia PR #116 is causing me serious headaches trying to send events down the pipe. Is it really necessary that an Event be build from a zbus::Message?

Based on what info we have, it should be fairly easy to have each event be a struct with the names of fields. For example:

struct TextCaretMovedEvent {
    pub id: AccessiblePrimitive,
    pub position: i32
}
// or
struct TextChangedEvent {
    pub id: AccessiblePrimitive,
    pub start: i32,
    pub length: i32,
    pub text: String,
}

This makes creating new events and sending them through a function like events::dispatch in Odilia much easier if the innermost type can be easily filled like this with simple public fields.

I also see that Cache::* and Available methods are written manually, which shouldn't be necessary under the new system. I'll basically wrire a macro that generates a slightly modified zvariant::Type implementation for each event.

It would also be nice if any event type could be trivially converted into a Signal that could be sent over DBus. This could simplify a lot of tests.

Imagine code like this in tests:

let a11y_bus = accessibility_,bus::connect().await?;
let caret_moved = CaretMovedEvent {
  id: AccessiblePrimitive {
    id: "/org/a11y/atspi/accessible/69",
    sender: ":1.1",
  },
  position: 420,
};
let msg = caret_moved.try_into()?;
a11y_bus.send_message(msg).await;