engine: Get rid of unnecessary templates to get nicer error messages

Now, instead of getting an error in the setter when we try to assign to
the mismatching type, we get an error in the code calling the setter
with the wrong type.

engine/engine.h

@@ -31,9 +31,9 @@ class Engine {
 
     void set_layout_width(int width);
 
-    void set_on_navigation_failure(auto cb) { on_navigation_failure_ = std::move(cb); }
-    void set_on_page_loaded(auto cb) { on_page_loaded_ = std::move(cb); }
-    void set_on_layout_updated(auto cb) { on_layout_update_ = std::move(cb); }
+    void set_on_navigation_failure(std::function<void(protocol::Error)> cb) { on_navigation_failure_ = std::move(cb); }
+    void set_on_page_loaded(std::function<void()> cb) { on_page_loaded_ = std::move(cb); }
+    void set_on_layout_updated(std::function<void()> cb) { on_layout_update_ = std::move(cb); }
 
     uri::Uri const &uri() const { return uri_; }
     protocol::Response const &response() const { return response_; }