OAuth Support (#53)

Rips out the old auth method, adds OAuth support. Fixes #51.

Tested Platforms:
- [x] Linux
    - [x] Flatpak
    - [x] Native (Nix)
- [x] Android
- [ ] Windows
- [ ] MacOS

Reviewed-on: Nexus/nexus#53

windows/runner/main.cpp

@@ -1,12 +1,17 @@
 #include <flutter/dart_project.h>
 #include <flutter/flutter_view_controller.h>
 #include <windows.h>
-
+#include "app_links/app_links_plugin_c_api.h"
 #include "flutter_window.h"
 #include "utils.h"
 
 int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
                       _In_ wchar_t *command_line, _In_ int show_command) {
+  // Forward the link to an existing instance, if any, then exit.
+  // You may ignore the result if you need to create another window.
+  if (SendAppLinkToInstance()) {
+    return EXIT_SUCCESS;
+  }
   // Attach to console when present (e.g., 'flutter run') or create a
   // new console when running with a debugger.
   if (!::AttachConsole(ATTACH_PARENT_PROCESS) && ::IsDebuggerPresent()) {