Move Win32 bindings into runtime state

src/platform_windows/windows_runtime_state.cpp

@@ -11,6 +11,7 @@ namespace {
 
 struct RetainedWindowsRuntimeState final {
     std::unique_ptr<App> owned_app;
+    App* app = nullptr;
     AppRuntime* runtime = nullptr;
     WacomTablet tablet;
 };
@@ -23,6 +24,30 @@ struct RetainedWindowsRuntimeState final {
 
 }
 
+void bind_app(App* app) noexcept
+{
+    auto& state = retained_runtime_state();
+    state.app = app;
+    App::I = app;
+}
+
+App* bound_app() noexcept
+{
+    return retained_runtime_state().app;
+}
+
+void release_bound_app() noexcept
+{
+    bind_app(nullptr);
+    bind_runtime(nullptr);
+    retained_owned_app().reset();
+}
+
+WacomTablet* bound_wacom_tablet() noexcept
+{
+    return &retained_wacom_tablet();
+}
+
 std::unique_ptr<App>& retained_owned_app() noexcept
 {
     return retained_runtime_state().owned_app;