qt: Add setting to prompt for user on game boot

Using the QtProfileSelectorDialog, this implementation is trivial. This mimics the real switch behavior of asking which user on every game boot, but it is default disabled as that might get inconvenient.

src/yuzu/ui_settings.h

@@ -40,6 +40,8 @@ struct Values {
     bool confirm_before_closing;
     bool first_start;
 
+    bool select_user_on_boot;
+
     // Discord RPC
     bool enable_discord_presence;