QGtkWindow: Improve the tick callback

* Don't unconditionally install a tick callback, these are only used for
  the QtQuick (or other requestUpdate) users, not the widgets case
* Remove the tick callback once window updates have ceased for a while,
  and reinstall it later.

This significantly decreases the amount of unnecessary wakeups on
otherwise idle gtkplatform clients.

For instance, telegram-desktop while idle (measured using powertop):
    before: 60-80 wakeups per second (consuming 160-220 mW of power)
    after: ~0.5-2 wakeups per second (consuming 0.8-2.5 mW of power)

src/platform-plugin/qgtkwindow.cpp

@@ -220,7 +220,6 @@ void QGtkWindow::create(Qt::WindowType windowType)
     g_signal_connect(m_window.get(), "size-allocate", G_CALLBACK(size_allocate_cb), this);
     g_signal_connect(m_window.get(), "delete-event", G_CALLBACK(delete_cb), this);
     g_signal_connect(m_window.get(), "window-state-event", G_CALLBACK(window_state_event_cb), this);
-    m_tick_callback = gtk_widget_add_tick_callback(m_window.get(), QGtkWindow::windowTickCallback, this, NULL);
 
     GtkWidget *vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
     gtk_container_add(GTK_CONTAINER(m_window.get()), vbox);

src/platform-plugin/qgtkwindow.h

@@ -38,7 +38,7 @@ QT_BEGIN_NAMESPACE
 
 class QTouchDevice;
 
-class QGtkWindow : public QPlatformWindow
+class QGtkWindow : public QObject, public QPlatformWindow
 {
 public:
     QGtkWindow(QWindow *window);
@@ -172,6 +172,8 @@ class QGtkWindow : public QPlatformWindow
     double m_initialZoom = 0;
     bool m_initialRotateSet = false;
     double m_initialRotate = 0;
+    bool m_hasTickCallback = false;
+    QTimer *m_cancelTickTimer = nullptr;
 };
 
 class QGtkCourierObject : public QObject

src/platform-plugin/qgtkwindow_render.cpp

@@ -29,6 +29,7 @@
 
 #include <QtCore/qloggingcategory.h>
 #include <QtCore/qthread.h>
+#include <QtCore/qtimer.h>
 #include <QtGui/private/qwindow_p.h>
 
 #include "CSystrace.h"
@@ -122,13 +123,43 @@ void QGtkWindow::onWindowTickCallback()
         m_wantsUpdate = false;
         TRACE_EVENT_ASYNC_END0("gfx", "QGtkWindow::requestUpdate", this);
         QWindowPrivate::get(window())->deliverUpdateRequest();
+    } else {
+        if (m_cancelTickTimer) {
+            return;
+        }
+        qCDebug(lcWindowRender) << "Preparing to remove tick callback" << this;
+        // Stop delivering ticks if update requests end for a long time.
+        m_cancelTickTimer = new QTimer(this);
+        m_cancelTickTimer->setInterval(1000);
+        m_cancelTickTimer->setSingleShot(true);
+        m_cancelTickTimer->start();
+        connect(m_cancelTickTimer, &QTimer::timeout, this, [=]() {
+            if (!m_wantsUpdate) {
+                qCDebug(lcWindowRender) << "Removing tick callback" << this;
+                gtk_widget_remove_tick_callback(m_window.get(), m_tick_callback);
+                m_tick_callback = 0;
+                m_hasTickCallback = false;
+                m_cancelTickTimer->deleteLater();
+                m_cancelTickTimer = nullptr;
+            }
+        });
     }
 }
 
 void QGtkWindow::requestUpdate()
 {
     TRACE_EVENT_ASYNC_BEGIN0("gfx", "QGtkWindow::requestUpdate", this);
     m_wantsUpdate = true;
+    if (!m_hasTickCallback) {
+        qCDebug(lcWindowRender) << "Installing tick callback" << this;
+        m_hasTickCallback = true;
+        m_tick_callback = gtk_widget_add_tick_callback(m_window.get(), QGtkWindow::windowTickCallback, this, NULL);
+    }
+    if (m_cancelTickTimer) {
+        m_cancelTickTimer->deleteLater();
+        m_cancelTickTimer = nullptr;
+        qCDebug(lcWindowRender) << "Cancelling remove of tick callback" << this;
+    }
 }
 
 QGtkCourierObject::QGtkCourierObject(QObject *parent)