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_widget.cpp

@@ -43,7 +43,7 @@ namespace Settings {
 namespace {
 
 QString SecretText;
-QMap<QString, base::lambda_copy<void()>> Codes;
+QMap<QString, base::lambda<void()>> Codes;
 
 void fillCodes() {
 	Codes.insert(qsl("debugmode"), []() {