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/ui/effects/widget_slide_wrap.h

@@ -33,7 +33,7 @@ public:
 	WidgetSlideWrap(QWidget *parent
 		, object_ptr<TWidget> entity
 		, style::margins entityPadding
-		, base::lambda<void()> &&updateCallback
+		, base::lambda<void()> updateCallback
 		, int duration = st::widgetSlideDuration);
 
 	void slideUp();
@@ -78,7 +78,7 @@ public:
 	WidgetSlideWrap(QWidget *parent
 		, object_ptr<Widget> entity
 		, style::margins entityPadding
-		, base::lambda<void()> &&updateCallback
+		, base::lambda<void()> updateCallback
 		, int duration = st::widgetSlideDuration) : WidgetSlideWrap<TWidget>(parent
 			, std::move(entity)
 			, entityPadding