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/window/themes/window_theme_warning.h

@@ -31,7 +31,7 @@ class WarningWidget : public TWidget {
 public:
 	WarningWidget(QWidget *parent);
 
-	void setHiddenCallback(base::lambda<void()> &&callback) {
+	void setHiddenCallback(base::lambda<void()> callback) {
 		_hiddenCallback = std::move(callback);
 	}