desktop: Change default value of Open Links to "Ask" (close #17432)

Change the default value of the "Open Links" setting from "Allow" to
"Ask". Having the default value of "Allow" may raise security concerns.
More details are in issue #17432, which this commit solves.

desktop/src/cli.rs

@@ -166,7 +166,7 @@ pub struct Opt {
     pub frame_rate: Option<f64>,
 
     /// The handling mode of links opening a new website.
-    #[clap(long, default_value = "allow")]
+    #[clap(long, default_value = "confirm")]
     pub open_url_mode: OpenURLMode,
 
     /// Provide a dummy (completely empty) External Interface to the movie.