[desktop] More qt_common reorganization (#3916)

Ported from QML branch.

Main "big" change is that EmuThread is now a shared state in QtCommon,
not individually managed/passed around by GRenderWindow and MainWindow.

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3916
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

src/qt_common/render/emu_thread.h

@@ -0,0 +1,97 @@
+// SPDX-FileCopyrightText: Copyright 2026 Eden Emulator Project
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#pragma once
+
+#include <QThread>
+#include "common/logging.h"
+#include "common/thread.h"
+
+namespace Core {
+class System;
+} // namespace Core
+
+namespace VideoCore {
+enum class LoadCallbackStage;
+} // namespace VideoCore
+
+class EmuThread final : public QThread {
+    Q_OBJECT
+
+public:
+    explicit EmuThread();
+    ~EmuThread() override;
+
+    /**
+     * Start emulation (on new thread)
+     * @warning Only call when not running!
+     */
+    void run() override;
+
+    /**
+     * Sets whether the emulation thread should run or not
+     * @param should_run Boolean value, set the emulation thread to running if true
+     */
+    void SetRunning(bool should_run) {
+        // TODO: Prevent other threads from modifying the state until we finish.
+        {
+            // Notify the running thread to change state.
+            std::unique_lock run_lk{m_should_run_mutex};
+            m_should_run = should_run;
+            m_should_run_cv.notify_one();
+        }
+
+        // Wait until paused, if pausing.
+        if (!should_run) {
+            m_stopped.Wait();
+        }
+    }
+
+    /**
+     * Check if the emulation thread is running or not
+     * @return True if the emulation thread is running, otherwise false
+     */
+    bool IsRunning() const {
+        return m_should_run;
+    }
+
+    /**
+     * Requests for the emulation thread to immediately stop running
+     */
+    void ForceStop() {
+        LOG_WARNING(Frontend, "Force stopping EmuThread");
+        m_stop_source.request_stop();
+    }
+
+private:
+    void EmulationPaused(std::unique_lock<std::mutex>& lk);
+    void EmulationResumed(std::unique_lock<std::mutex>& lk);
+
+private:
+    std::stop_source m_stop_source;
+    std::mutex m_should_run_mutex;
+    std::condition_variable_any m_should_run_cv;
+    Common::Event m_stopped;
+    bool m_should_run{true};
+
+signals:
+    /**
+     * Emitted when the CPU has halted execution
+     *
+     * @warning When connecting to this signal from other threads, make sure to specify either
+     * Qt::QueuedConnection (invoke slot within the destination object's message thread) or even
+     * Qt::BlockingQueuedConnection (additionally block source thread until slot returns)
+     */
+    void DebugModeEntered();
+
+    /**
+     * Emitted right before the CPU continues execution
+     *
+     * @warning When connecting to this signal from other threads, make sure to specify either
+     * Qt::QueuedConnection (invoke slot within the destination object's message thread) or even
+     * Qt::BlockingQueuedConnection (additionally block source thread until slot returns)
+     */
+    void DebugModeLeft();
+
+    void LoadProgress(VideoCore::LoadCallbackStage stage, std::size_t value, std::size_t total);
+};