Add EventFilterInterface and port DockRegistry to it

Some classes wont have any Qt in it, so they need another way
to install global event filters. Added API to Platform.h for this
purpose. Implemented as regular event filter in Platform_qt.

src/qtcommon/Platform_qt.cpp

@@ -12,6 +12,7 @@
 #include "Platform_qt.h"
 #include "kddockwidgets/KDDockWidgets.h"
 #include "kddockwidgets/Window_qt.h"
+#include "kddockwidgets/EventFilterInterface.h"
 #include "private/Platform_p.h"
 
 #include <QWindow>
@@ -101,7 +102,8 @@ EventFilter::~EventFilter() = default;
 class Platform_qt::GlobalEventFilter : public QObject
 {
 public:
-    GlobalEventFilter()
+    GlobalEventFilter(Platform_qt *qq)
+        : q(qq)
     {
         if (qGuiApp)
             qGuiApp->installEventFilter(this);
@@ -109,6 +111,9 @@ public:
 
     bool eventFilter(QObject *o, QEvent *ev) override
     {
+        if (ev->type() == QEvent::Expose)
+            return handleExpose(o);
+
         auto view = Platform::instance()->qobjectAsView(o);
         if (!view)
             return false;
@@ -123,14 +128,32 @@ public:
         return false;
     }
 
+    bool handleExpose(QObject *o)
+    {
+        if (q->d->m_globalEventFilters.empty())
+            return false;
+
+        auto window = Platform::instance()->qobjectAsWindow(o);
+        if (!window)
+            return false;
+
+        for (EventFilterInterface *filter : qAsConst(q->d->m_globalEventFilters)) {
+            if (filter->onExposeEvent(window))
+                return true;
+        }
+
+        return false;
+    }
+
     ~GlobalEventFilter() override;
     bool m_isProcessingAppQuitEvent = false;
+    Platform_qt *const q;
 };
 
 Platform_qt::GlobalEventFilter::~GlobalEventFilter() = default;
 
 Platform_qt::Platform_qt()
-    : m_globalEventFilter(new GlobalEventFilter())
+    : m_globalEventFilter(new GlobalEventFilter(this))
 {
     if (!qGuiApp)
         qWarning() << "Please call KDDockWidgets::initPlatform() after QGuiApplication";
@@ -308,7 +331,7 @@ bool Platform_qt::isGammaray()
 }
 
 Platform_qt::Platform_qt(int &argc, char **argv)
-    : m_globalEventFilter(new GlobalEventFilter())
+    : m_globalEventFilter(new GlobalEventFilter(this))
 {
     // This CTOR is called before we have a QApplication