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.

App/Source/HelloWorld/HelloWorld.cpp

@@ -1,17 +1,23 @@
+#include "app/AppManifest.h"
 #include "lvgl.h"
 #include "lvgl/Toolbar.h"
 
-static void onShow(tt::app::AppContext& context, lv_obj_t* parent) {
-    lv_obj_t* toolbar = tt::lvgl::toolbar_create(parent, context);
-    lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
+using namespace tt::app;
 
-    lv_obj_t* label = lv_label_create(parent);
-    lv_label_set_text(label, "Hello, world!");
-    lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);
-}
+class HelloWorldApp : public App {
 
-extern const tt::app::AppManifest hello_world_app = {
+    void onShow(AppContext& context, lv_obj_t* parent) override {
+        lv_obj_t* toolbar = tt::lvgl::toolbar_create(parent, context);
+        lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
+
+        lv_obj_t* label = lv_label_create(parent);
+        lv_label_set_text(label, "Hello, world!");
+        lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);
+    }
+};
+
+extern const AppManifest hello_world_app = {
     .id = "HelloWorld",
     .name = "Hello World",
-    .onShow = onShow,
+    .createApp = create<HelloWorldApp>
 };