Refactor app launching (#174)

- Refactor the way apps work: Instead of a C interface, they are now C++ classes. The main reasoning is that attaching data to an app was cumbersome. Having different implementations for different kinds of apps was cumbersome too. (3 or 4 layers of manifest nesting for the TactilityC project)
- External apps are still written in C, but they get a createData/destroyData in their manifest, so:
- External apps now have their own manifest.
- All functions in the original AppManifest are removed and replaced by a single `createApp` function
- External apps now automatically register (each app individually!) when they run the first time. As a side-effect they become visible in the `AppList` app!
- Adapted all apps for the new interface.
- Adapted all internal logic for these changes (Gui, ViewPort, Loader, AppContext, AppInstance, etc.)
- Rewrote parts of Loader to use std::shared_ptr to make the code much safer.
- Added a refcount check for the `AppInstance` and `App` at the end of their lifecycle. Show warning if refcount is too high.

Tactility/Source/service/loader/Loader.h

@@ -21,8 +21,11 @@ void startApp(const std::string& id, const std::shared_ptr<const Bundle>& _Nulla
 /** @brief Stop the currently showing app. Show the previous app if any app was still running. */
 void stopApp();
 
+/** @return the currently running app context (it is only ever null before the splash screen is shown) */
+std::shared_ptr<app::AppContext> _Nullable getCurrentAppContext();
+
 /** @return the currently running app (it is only ever null before the splash screen is shown) */
-app::AppContext* _Nullable getCurrentApp();
+std::shared_ptr<app::App> _Nullable getCurrentApp();
 
 /**
  * @brief PubSub for LoaderEvent