Add isOnDesktop() overload that takes VirtualDesktop

int-based desktop ids are unreliable, but VirtualDesktop are not. So far
virtual desktop objects aren't used widely in kwin, but it's a good start.

src/toplevel.cpp

@@ -691,14 +691,19 @@ QMargins Toplevel::frameMargins() const
     return QMargins();
 }
 
+bool Toplevel::isOnDesktop(VirtualDesktop *desktop) const
+{
+    return isOnAllDesktops() || desktops().contains(desktop);
+}
+
 bool Toplevel::isOnDesktop(int d) const
 {
-    return isOnAllDesktops() || desktops().contains(VirtualDesktopManager::self()->desktopForX11Id(d));
+    return isOnDesktop(VirtualDesktopManager::self()->desktopForX11Id(d));
 }
 
 bool Toplevel::isOnCurrentDesktop() const
 {
-    return isOnDesktop(VirtualDesktopManager::self()->current());
+    return isOnDesktop(VirtualDesktopManager::self()->currentDesktop());
 }
 
 } // namespace

src/toplevel.h

@@ -409,6 +409,7 @@ public:
     virtual int desktop() const = 0;
     virtual QVector<VirtualDesktop *> desktops() const = 0;
     virtual QStringList activities() const = 0;
+    bool isOnDesktop(VirtualDesktop *desktop) const;
     bool isOnDesktop(int d) const;
     bool isOnActivity(const QString &activity) const;
     bool isOnCurrentDesktop() const;