Made base::lambda a copyable value type.

Now base::lambda can be copied and can wrap any immutable lambda.
For mutable lambdas there is base::lambda_once (which you're
supposed to call only once to pass data through lambda captures,
for example by using std::unique_ptr). Generally base::lambda is
passed by value and base::lambda_once is passed by rvalue reference.

Telegram/SourceFiles/settings/settings_layer.h

@@ -51,7 +51,7 @@ class Layer : public LayerWidget {
 public:
 	Layer();
 
-	void setCloseClickHandler(base::lambda<void()> &&callback);
+	void setCloseClickHandler(base::lambda<void()> callback);
 	void resizeToWidth(int newWidth, int newContentLeft);
 
 protected: