[feat] Add Window::title getter

[tauri-bot]

This issue has been upstreamed from tauri-apps/tauri#5023

Describe the problem

It's tricky when making custom title bars for a window—or trying to update the window title with something based on the current title—to actually pin down in code what the current title is.

Describe the solution you'd like

On the appWindow level, we have the ability to call appWindow.setTitle("..."). It would be good if we could get a reference to the current title here as well. A property of appWindow.title would probably be best, but exposing it through appWindow.getTitle() would also work fine.

Alternatives considered

No response

Additional context

No response