Fix: fixes a bug that crashed ncspot.

Like mentioned in the corresponding issue by ayushjaipuriyar, ncspot
would crash when the queue was empty, shuffled, and played. This commit
fixes that, together with some minor clippy fixes.

src/ui/cover.rs

@@ -204,7 +204,7 @@ impl View for CoverView {
             }
         });
 
-        let cover_url = self.queue.get_current().map(|t| t.cover_url()).flatten();
+        let cover_url = self.queue.get_current().and_then(|t| t.cover_url());
 
         if let Some(url) = cover_url {
             self.draw_cover(url, printer.offset, printer.size);

src/ui/layout.rs

@@ -143,6 +143,7 @@ impl Layout {
         self.get_focussed_stack_mut().map(|stack| stack.pop());
     }
 
+    #[allow(clippy::borrowed_box)]
     fn get_current_screen(&self) -> Option<&Box<dyn ViewExt>> {
         self.focus
             .as_ref()
@@ -168,6 +169,7 @@ impl Layout {
             .unwrap_or(false)
     }
 
+    #[allow(clippy::borrowed_box)]
     fn get_top_view(&self) -> Option<&Box<dyn ViewExt>> {
         let focussed_stack = self.get_focussed_stack();
         if focussed_stack.map(|s| s.len()).unwrap_or_default() > 0 {