Update Flutter to Version 3.16.5 (#106)

Update the used Flutter version to 3.16.5 and the used packages to their
latest version.

The Supabase package contained some breaking changes:
- `functionUrl` is not exported anymore, so that it must be generated by
  ourselfs
- `Provider` was renamed to `OAuthProvider`
- The `signInWithApple` method was removed and is now implemented by us
  via the `sign_in_with_apple` package.

We also renamed the `DesktopLoginManager` to `DesktopSignInManager` to
use the same naming as in other places of the app, where we are always
using sign in and not login.

app/linux/flutter/generated_plugin_registrant.cc

@@ -6,6 +6,7 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <gtk/gtk_plugin.h>
 #include <media_kit_libs_linux/media_kit_libs_linux_plugin.h>
 #include <media_kit_video/media_kit_video_plugin.h>
 #include <screen_retriever/screen_retriever_plugin.h>
@@ -13,6 +14,9 @@
 #include <window_manager/window_manager_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
+  g_autoptr(FlPluginRegistrar) gtk_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "GtkPlugin");
+  gtk_plugin_register_with_registrar(gtk_registrar);
   g_autoptr(FlPluginRegistrar) media_kit_libs_linux_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "MediaKitLibsLinuxPlugin");
   media_kit_libs_linux_plugin_register_with_registrar(media_kit_libs_linux_registrar);

app/linux/flutter/generated_plugins.cmake

@@ -3,6 +3,7 @@
 #
 
 list(APPEND FLUTTER_PLUGIN_LIST
+  gtk
   media_kit_libs_linux
   media_kit_video
   screen_retriever