Version independent external apps (standalone apps) (#2145)

This pull requests adds a new type of external app to the firmware: The standalone app.

Pros:

Will work after an upgrade.
Size of image is only limited by shared heap size of M0 (application) (64kb total).
Cons:

No full access to all functions in the main firmware. One well defined (and versioned) API handles all communication.
The Pacman app was converted to be the first the the new kind.

firmware/application/ui_external_items_menu_loader.hpp

@@ -26,6 +26,7 @@
 #include "ui.hpp"
 #include "ui_navigation.hpp"
 #include "external_app.hpp"
+#include "standalone_app.hpp"
 
 #include "file.hpp"
 
@@ -57,6 +58,7 @@ class ExternalItemsMenuLoader {
     static std::vector<GridItem> load_external_items(app_location_t, NavigationView&);
     ExternalItemsMenuLoader() = delete;
     static bool run_external_app(ui::NavigationView&, std::filesystem::path);
+    static bool run_standalone_app(ui::NavigationView&, std::filesystem::path);
     static void load_all_external_items_callback(std::function<void(AppInfoConsole&)> callback);
 
    private: