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/overview/overview_layout.cpp

@@ -156,7 +156,7 @@ public:
 private:
 	void startAnimation();
 
-	base::lambda_copy<void()> _updateCallback;
+	base::lambda<void()> _updateCallback;
 	Ui::RoundCheckbox _check;
 
 	Animation _pression;