remember view stacks separately for screens

Previously, the stack would be reset when switching between screens.
With the new search flow this means that search results are lost, e.g. when switching
from search results to the queue and back.

src/ui/library.rs

@@ -54,6 +54,10 @@ impl ViewWrapper for LibraryView {
 }
 
 impl ViewExt for LibraryView {
+    fn title(&self) -> String {
+        "Library".to_string()
+    }
+
     fn on_command(&mut self, s: &mut Cursive, cmd: &Command) -> Result<CommandResult, String> {
         self.tabs.on_command(s, cmd)
     }