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/app/i2cscanner/I2cScannerPrivate.h

@@ -19,17 +19,7 @@ enum ScanState {
 };
 
 struct Data {
-    // Core
-    Mutex mutex = Mutex(Mutex::Type::Recursive);
-    std::unique_ptr<Timer> scanTimer = nullptr;
-    // State
-    ScanState scanState;
-    i2c_port_t port = I2C_NUM_0;
-    std::vector<uint8_t> scannedAddresses;
-    // Widgets
-    lv_obj_t* scanButtonLabelWidget = nullptr;
-    lv_obj_t* portDropdownWidget = nullptr;
-    lv_obj_t* scanListWidget = nullptr;
+
 };
 
 void onScanTimerFinished(std::shared_ptr<Data> data);