Simplify code that destroys plugins and color manager

This reduces some code duplication and ensures that plugins and the
color manager are destroyed while Platform is still valid.

autotests/integration/kwin_wayland_test.cpp

@@ -80,7 +80,6 @@ WaylandTestApplication::~WaylandTestApplication()
     if (effects) {
         static_cast<EffectsHandlerImpl*>(effects)->unloadAllEffects();
     }
-    destroyPlugins();
     delete m_xwayland;
     m_xwayland = nullptr;
     destroyWorkspace();
@@ -90,7 +89,6 @@ WaylandTestApplication::~WaylandTestApplication()
     }
     waylandServer()->terminateClientConnections();
     destroyCompositor();
-    destroyColorManager();
 }
 
 void WaylandTestApplication::performStartup()

src/main.cpp

@@ -148,6 +148,8 @@ void Application::start()
 Application::~Application()
 {
     delete options;
+    destroyPlugins();
+    destroyColorManager();
     destroyAtoms();
     destroyPlatform();
 }

src/main_wayland.cpp

@@ -136,7 +136,6 @@ ApplicationWayland::~ApplicationWayland()
     if (effects) {
         static_cast<EffectsHandlerImpl*>(effects)->unloadAllEffects();
     }
-    destroyPlugins();
     delete m_xwayland;
     m_xwayland = nullptr;
     destroyWorkspace();
@@ -147,7 +146,6 @@ ApplicationWayland::~ApplicationWayland()
     }
     waylandServer()->terminateClientConnections();
     destroyCompositor();
-    destroyColorManager();
 }
 
 void ApplicationWayland::performStartup()

src/main_x11.cpp

@@ -190,8 +190,6 @@ void ApplicationX11::setReplace(bool replace)
 void ApplicationX11::lostSelection()
 {
     sendPostedEvents();
-    destroyPlugins();
-    destroyColorManager();
     destroyCompositor();
     destroyWorkspace();
     // Remove windowmanager privileges