[decorations] Bring back option NoPlugin

The option NoPlugin allows to disable window decoration plugins. It
used to exist with the old decoration library but got unfortunately
lost during the switch.

This change brings back the option and also allows runtime changes
of the option.

REVIEW: 124708

decorations/decorationbridge.cpp

@@ -56,6 +56,7 @@ DecorationBridge::DecorationBridge(QObject *parent)
     , m_factory(nullptr)
     , m_blur(false)
     , m_settings()
+    , m_noPlugin(false)
 {
 }
 
@@ -69,6 +70,11 @@ static QString readPlugin()
     return KSharedConfig::openConfig(KWIN_CONFIG)->group(s_pluginName).readEntry("library", s_defaultPlugin);
 }
 
+static bool readNoPlugin()
+{
+    return KSharedConfig::openConfig(KWIN_CONFIG)->group(s_pluginName).readEntry("NoPlugin", false);
+}
+
 QString DecorationBridge::readTheme() const
 {
     return KSharedConfig::openConfig(KWIN_CONFIG)->group(s_pluginName).readEntry("theme", m_defaultTheme);
@@ -76,6 +82,10 @@ QString DecorationBridge::readTheme() const
 
 void DecorationBridge::init()
 {
+    m_noPlugin = readNoPlugin();
+    if (m_noPlugin) {
+        return;
+    }
     m_plugin = readPlugin();
     m_settings = QSharedPointer<KDecoration2::DecorationSettings>::create(this);
     initPlugin();
@@ -118,6 +128,23 @@ static void recreateDecorations()
 
 void DecorationBridge::reconfigure()
 {
+    if (m_noPlugin != readNoPlugin()) {
+        m_noPlugin = !m_noPlugin;
+        // no plugin setting changed
+        if (m_noPlugin) {
+            // decorations disabled now
+            m_plugin = QString();
+            delete m_factory;
+            m_factory = nullptr;
+            m_settings.clear();
+        } else {
+            // decorations enabled now
+            init();
+        }
+        recreateDecorations();
+        return;
+    }
+
     const QString newPlugin = readPlugin();
     if (newPlugin != m_plugin) {
         // plugin changed, recreate everything
@@ -200,6 +227,9 @@ void DecorationBridge::update(KDecoration2::Decoration *decoration, const QRect
 
 KDecoration2::Decoration *DecorationBridge::createDecoration(Client *client)
 {
+    if (m_noPlugin) {
+        return nullptr;
+    }
     if (!m_factory) {
         return nullptr;
     }

decorations/decorationbridge.h

@@ -78,6 +78,7 @@ private:
     QString m_defaultTheme;
     QString m_theme;
     QSharedPointer<KDecoration2::DecorationSettings> m_settings;
+    bool m_noPlugin;
     KWIN_SINGLETON(DecorationBridge)
 };
 } // Decoration