Fix DesktopGrid drag on X11

Summary:
EffectsAPI explicitly says:
"On X11, the window will end up on the last window in the list" and
DesktopGrid reliaed on that.

Using the last makes sense as it means the
enterDesktop method will work for both.

Somehow in the refactors AbstractClient ended up doing the opposite.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18339

abstract_client.cpp

@@ -501,7 +501,7 @@ void AbstractClient::setDesktops(QVector<VirtualDesktop*> desktops)
 {
     //on x11 we can have only one desktop at a time
     if (kwinApp()->operationMode() == Application::OperationModeX11 && desktops.size() > 1) {
-        desktops = QVector<VirtualDesktop*>({desktops.first()});
+        desktops = QVector<VirtualDesktop*>({desktops.last()});
     }
 
     if (desktops == m_desktops) {

abstract_client.h

@@ -430,6 +430,11 @@ public:
     void setDesktop(int);
     void enterDesktop(VirtualDesktop *desktop);
     void leaveDesktop(VirtualDesktop *desktop);
+
+    /**
+     * Set the window as being on the attached list of desktops
+     * On X11 it will be set to the last entry
+     */
     void setDesktops(QVector<VirtualDesktop *> desktops);
 
     int desktop() const override {