Wrap the sidebar in a ScrolledWindow

Previously, the sidebar did not have scrolling enabled.  This resulted
in poor behavior in very short windows, such as landscape on phones,
causing the bottom to be cut off.  This commit fixes it by wrapping the
sidebar's ListBox in a ScrolledWindow.

data/gtk/window.blp

@@ -89,6 +89,7 @@ template $CartridgesWindow : Adw.ApplicationWindow {
                   }
                 }
 
+                ScrolledWindow {
                   ListBox sidebar {
                     Box all_games_row_box {
                       margin-top: 12;
@@ -146,6 +147,7 @@ template $CartridgesWindow : Adw.ApplicationWindow {
                   }
                 }
               }
+            }
 
             Adw.ToolbarView library_view {