Browser: Convert search-engines file loading to Core::Stream :^)

Wrapped it in a method so we can take advantage of TRY(). I chose not to
make failure here stop the Browser process, but just to cancel loading
any more search engines.

Userland/Applications/Browser/BrowserWindow.h

@@ -41,6 +41,7 @@ private:
     explicit BrowserWindow(CookieJar&, URL);
 
     void build_menus();
+    ErrorOr<void> load_search_engines(GUI::Menu& settings_menu);
     void set_window_title_for_tab(Tab const&);
 
     RefPtr<GUI::Action> m_go_back_action;