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.

TactilityCore/Source/StringUtils.cpp

@@ -78,4 +78,13 @@ std::string join(const std::vector<std::string>& input, const std::string& delim
     return stream.str();
 }
 
+std::string removeFileExtension(const std::string& input) {
+    auto index = input.find('.');
+    if (index != std::string::npos) {
+        return input.substr(0, index);
+    } else {
+        return input;
+    }
+}
+
 } // namespace