Use signal/slot approach for pluginLoadedChanged

Per @Trinitou feedback:

1. pluginLoadedChanged is now a signal of PluginHost

2. Connect MainWindow:: updatePluginMenuItems to this signal

3. Rename updateMenuItems to updatePluginMenuItems (so it specifies it's these are plugin related menu items)

4. Remove i_prefix in function parameter to follow code style

5. Use default argument false for updatePluginMenuItems

host/main-window.cc

@@ -99,15 +99,18 @@ void MainWindow::createMenu() {
    connect(
       helpMenu->addAction(tr("About")), &QAction::triggered, this, &MainWindow::showAboutDialog);
 
-   updateMenuItems();
+   updatePluginMenuItems();
+
+   assert(_application.engine());
+   connect(&_application.engine()->pluginHost(), &PluginHost::pluginLoadedChanged, this, &MainWindow::updatePluginMenuItems);
 }
 
-void MainWindow::updateMenuItems() {
-   _loadPluginPresetAction->setEnabled(_pluginLoaded);
-   _showPluginParametersAction->setEnabled(_pluginLoaded);
-   _showPluginQuickControlsAction->setEnabled(_pluginLoaded);
-   _togglePluginWindowVisibilityAction->setEnabled(_pluginLoaded);
-   _recreatePluginWindowAction->setEnabled(_pluginLoaded);
+void MainWindow::updatePluginMenuItems(bool const pluginLoaded /* = false */ ) {
+   _loadPluginPresetAction->setEnabled(pluginLoaded);
+   _showPluginParametersAction->setEnabled(pluginLoaded);
+   _showPluginQuickControlsAction->setEnabled(pluginLoaded);
+   _togglePluginWindowVisibilityAction->setEnabled(pluginLoaded);
+   _recreatePluginWindowAction->setEnabled(pluginLoaded);
 }
 
 void MainWindow::showSettingsDialog() {
@@ -137,11 +140,6 @@ void MainWindow::resizePluginView(int width, int height) {
    adjustSize();
 }
 
-void MainWindow::onPluginLoadChange(bool const i_pluginLoaded) {
-   _pluginLoaded = i_pluginLoaded;
-   updateMenuItems();
-}
-
 void MainWindow::loadNativePluginPreset() {
    auto file = QFileDialog::getOpenFileName(this, tr("Load Plugin Native Preset"));
    if (file.isEmpty())

host/main-window.hh

@@ -25,7 +25,6 @@ public:
    void showPluginParametersWindow();
    void showPluginQuickControlsWindow();
    void resizePluginView(int width, int height);
-   void onPluginLoadChange(bool i_pluginLoaded);
 
    void showPluginWindow() { _pluginViewWidget->show(); }
 
@@ -42,7 +41,7 @@ private:
    void togglePluginWindowVisibility();
    void recreatePluginWindow();
    void showAboutDialog();
-   void updateMenuItems();
+   void updatePluginMenuItems(bool pluginLoaded = false);
 
    Application &_application;
    QWindow *_pluginViewWindow = nullptr;
@@ -54,8 +53,6 @@ private:
    QAction *_togglePluginWindowVisibilityAction = nullptr;
    QAction *_recreatePluginWindowAction = nullptr;
 
-   bool _pluginLoaded = false;
-
    PluginParametersWidget *_pluginParametersWidget = nullptr;
    PluginQuickControlsWidget *_pluginRemoteControlsWidget = nullptr;
 };

host/plugin-host.cc

@@ -148,15 +148,15 @@ bool PluginHost::load(const QString &path, int pluginIndex) {
    scanParams();
    scanQuickControls();
 
-   Application::instance().mainWindow()->onPluginLoadChange(true);
+   pluginLoadedChanged(true);
 
    return true;
 }
 
 void PluginHost::unload() {
    checkForMainThread();
 
-   Application::instance().mainWindow()->onPluginLoadChange(false);
+   pluginLoadedChanged(false);
 
    if (!_library.isLoaded())
       return;

host/plugin-host.hh

@@ -92,6 +92,7 @@ signals:
    void quickControlsPagesChanged();
    void quickControlsSelectedPageChanged();
    void paramAdjusted(clap_id paramId);
+   void pluginLoadedChanged(bool pluginLoaded);
 
 protected:
    /////////////////////////