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.

ExternalApps/HelloWorld/main/Source/main.c

@@ -5,7 +5,7 @@
  * Note: LVGL and Tactility methods need to be exposed manually from TactilityC/Source/tt_init.cpp
  * Only C is supported for now (C++ symbols fail to link)
  */
-static void onShow(AppContextHandle context, lv_obj_t* parent) {
+static void onShow(AppContextHandle context, void* data, lv_obj_t* parent) {
     lv_obj_t* toolbar = tt_lvgl_toolbar_create(parent, context);
     lv_obj_align(toolbar, LV_ALIGN_TOP_MID, 0, 0);
 
@@ -14,15 +14,12 @@ static void onShow(AppContextHandle context, lv_obj_t* parent) {
     lv_obj_align(label, LV_ALIGN_CENTER, 0, 0);
 }
 
+ExternalAppManifest manifest = {
+    .name = "Hello World",
+    .onShow = onShow
+};
+
 int main(int argc, char* argv[]) {
-    tt_set_app_manifest(
-        "Hello World",
-        NULL,
-        NULL,
-        NULL,
-        onShow,
-        NULL,
-        NULL
-    );
+    tt_app_register(&manifest);
     return 0;
 }