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/layerwidget.cpp

@@ -45,7 +45,7 @@ public:
 	BackgroundWidget(QWidget *parent) : TWidget(parent) {
 	}
 
-	void setDoneCallback(base::lambda<void()> &&callback) {
+	void setDoneCallback(base::lambda<void()> callback) {
 		_doneCallback = std::move(callback);
 	}