Split Linux platform target and move runtime/platform state

src/app_runtime.h

@@ -46,6 +46,8 @@ public:
     void notify_ui_worker() noexcept;
     void prepared_file_task(std::function<void()> task);
     void canvas_async_task(std::function<void()> task);
+    void main_thread_task(std::packaged_task<void()> task);
+    void drain_main_thread_tasks();
 
     void render_thread_tick(App& app);
     void render_thread_main(App& app, std::stop_token stop_token);
@@ -221,6 +223,9 @@ private:
     std::jthread canvas_async_worker_;
     bool canvas_async_running_ = true;
 
+    std::deque<std::packaged_task<void()>> main_thread_tasklist_;
+    std::mutex main_thread_task_mutex_;
+
     std::deque<AppTask> render_tasklist_;
     std::mutex render_task_mutex_;
     std::condition_variable render_cv_;