Opens a system prompt.
If prompter_name is null, then the default system prompter is used.
Most system prompters only allow showing one prompt at a time, and if another prompt is shown then this method will block for up to
timeout_seconds seconds. If timeout_seconds is equal to -1, then this will block indefinitely until the prompt
can be opened. If timeout_seconds expires, then this function will fail with a
gcr_system_prompt_in_progress error.
| prompter_name |
the prompter dbus name |
| timeout_seconds |
the number of seconds to wait to access the prompt, or -1 |
| cancellable |
optional cancellation object |
|
the prompt, or null if prompt could not be opened |