cleanup, timermanager in App is a dependency that takes timers again.

App now adds already constructed Timers
Timer is now an Invoker that takes a weak_ptr to a steppable with a
request()rather than a bool callback.
should eventually be changed to composition

src/Invoker.hpp

@@ -0,0 +1,19 @@
+#ifndef INVOKER_HPP
+#define INVOKER_HPP
+
+#include "steppable.hpp"
+#include <memory>
+class Invoker {
+public:
+  virtual ~Invoker() = default;
+  std::weak_ptr<Steppable> target;
+
+  void set_target(std::shared_ptr<Steppable> t) { target = t; }
+  void request() {
+    if (auto sp = target.lock()) {
+      sp->step();
+    }
+  }
+};
+
+#endif