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/screenshot/ScreenshotTask.cpp

@@ -87,9 +87,9 @@ void ScreenshotTask::taskMain() {
                 }
             }
         } else if (work.type == TASK_WORK_TYPE_APPS) {
-            app::AppContext* _Nullable app = loader::getCurrentApp();
-            if (app) {
-                const app::AppManifest& manifest = app->getManifest();
+            auto appContext = loader::getCurrentAppContext();
+            if (appContext != nullptr) {
+                const app::AppManifest& manifest = appContext->getManifest();
                 if (manifest.id != last_app_id) {
                     kernel::delayMillis(100);
                     last_app_id = manifest.id;