Move implementation of clientPos() to AbstractClient

The Client's variant works for both Client and ShellClient.

abstract_client.h

@@ -284,6 +284,10 @@ public:
 
     bool wantsTabFocus() const;
 
+    QPoint clientPos() const override {
+        return QPoint(borderLeft(), borderTop());
+    }
+
     virtual void updateMouseGrab();
     virtual QString caption(bool full = true, bool stripped = false) const = 0;
     virtual bool isCloseable() const = 0;

client.h

@@ -145,7 +145,6 @@ public:
     QSize minSize() const override;
     QSize maxSize() const override;
     QSize basicUnit() const;
-    virtual QPoint clientPos() const; // Inside of geometry()
     virtual QSize clientSize() const;
     QPoint inputPos() const { return input_offset; } // Inside of geometry()
 
@@ -719,11 +718,6 @@ inline bool Client::isManaged() const
     return m_managed;
 }
 
-inline QPoint Client::clientPos() const
-{
-    return QPoint(borderLeft(), borderTop());
-}
-
 inline QSize Client::clientSize() const
 {
     return client_size;

shell_client.cpp

@@ -195,11 +195,6 @@ QStringList ShellClient::activities() const
     return QStringList();
 }
 
-QPoint ShellClient::clientPos() const
-{
-    return QPoint(0, 0);
-}
-
 QPoint ShellClient::clientContentPos() const
 {
     return -1 * clientPos();

shell_client.h

@@ -43,7 +43,6 @@ public:
     virtual ~ShellClient();
 
     QStringList activities() const override;
-    QPoint clientPos() const override;
     QPoint clientContentPos() const override;
     QSize clientSize() const override;
     QRect transparentRect() const override;