Add const method variants to Xcb::Wrapper

Adding a const variant for ::data() to remove an unexpected copy.
To complete also the bool cast operator and isNull get a const
variant.

REVIEW: 117469

workspace.cpp

@@ -351,7 +351,7 @@ void Workspace::init()
                     createUnmanaged(wins[i]);
             } else if (attr->map_state != XCB_MAP_STATE_UNMAPPED) {
                 if (Application::wasCrash()) {
-                    fixPositionAfterCrash(wins[i], windowGeometries[i].data());
+                    fixPositionAfterCrash(wins[i], windowGeometries.at(i).data());
                 }
 
                 // ### This will request the attributes again

xcbutils.h

@@ -102,13 +102,24 @@ public:
         getReply();
         return m_reply == NULL;
     }
+    inline bool isNull() const {
+        const_cast<Wrapper*>(this)->getReply();
+        return m_reply == NULL;
+    }
     inline operator bool() {
         return !isNull();
     }
+    inline operator bool() const {
+        return !isNull();
+    }
     inline const Reply *data() {
         getReply();
         return m_reply;
     }
+    inline const Reply *data() const {
+        const_cast<Wrapper*>(this)->getReply();
+        return m_reply;
+    }
     inline WindowId window() const {
         return m_window;
     }