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/Private/service/gui/Gui_i.h

@@ -4,9 +4,8 @@
 #include "Mutex.h"
 #include "Pubsub.h"
 #include "service/gui/Gui.h"
-#include "service/gui/ViewPort.h"
-#include "service/gui/ViewPort_i.h"
 #include <cstdio>
+#include <lvgl.h>
 
 namespace tt::service::gui {
 
@@ -27,7 +26,7 @@ struct Gui {
     lv_obj_t* statusbarWidget = nullptr;
 
     // App-specific
-    ViewPort* appViewPort = nullptr;
+    std::shared_ptr<app::AppContext> appToRender = nullptr;
 
     lv_obj_t* _Nullable keyboard = nullptr;
     lv_group_t* keyboardGroup = nullptr;

Tactility/Private/service/gui/ViewPort_i.h

@@ -1,23 +0,0 @@
-#pragma once
-
-#include "service/gui/ViewPort.h"
-
-namespace tt::service::gui {
-
-/** Process draw call. Calls onShow callback.
- * To be used by GUI, called on redraw.
- *
- * @param      view_port  ViewPort instance
- * @param      canvas     canvas to draw at
- */
-void view_port_show(ViewPort* view_port, lv_obj_t* parent);
-
-/**
- * Process draw clearing call. Calls on_hdie callback.
- * To be used by GUI, called on redraw.
- *
- * @param view_port
- */
-void view_port_hide(ViewPort* view_port);
-
-} // namespace