Playlist with multiple, mixed items

Prior to this commit, projects/playlists were limited to folders even if
file URLs were passed as arguments. With this commit both files and
folders are added to projects from the command line.

This doesn't affect opening a file from the GUI yet, but this patch
implements the required plumbing for that as well.

src/argparse.rs

@@ -8,8 +8,10 @@ use url::Url;
 
 #[derive(Debug, Default)]
 pub struct Arguments {
-    /// URLs to play with associated metadata
-    pub urls: Vec<Url>,
+    /// Files or directory URLs to play
+    pub urls: Option<Vec<Url>>,
+    /// Single URL only
+    pub url_opt: Option<Url>,
 }
 
 impl Arguments {
@@ -31,7 +33,17 @@ impl Arguments {
             warn!("Unused argument: {arg:?}");
         }
 
-        Ok(Arguments { urls })
+        if urls.len() > 1 {
+            Ok(Arguments {
+                urls: Some(urls),
+                ..Default::default()
+            })
+        } else {
+            Ok(Arguments {
+                url_opt: urls.into_iter().next(),
+                ..Default::default()
+            })
+        }
     }
 }