gui: change combiner for signals to optional_last_value

optional_last_value, which does not throw, has replaced optional_value as
boost's default combiner. Besides being better supported, it also doesn't
trigger gcc's -Wmaybe-unitialized warning, presumably because exceptions no
longer bubble-up out of signals:

```bash
boost/signals2/last_value.hpp:54:36: warning: '*((void*)& value +1)' may be used uninitialized in this function [-Wmaybe-uninitialized]
	if(value) return value.get();
```

The change in default happened in Boost 1.39.0 (along with the
introduction of the signals 2 library. More information is available here:

https://www.boost.org/doc/libs/1_73_0/doc/html/signals2/rationale.html#id-1.3.36.9.4

and here:

https://www.boost.org/doc/libs/1_73_0/doc/html/boost/signals2/optional_last_value.html

Co-authored-by: fanquake <fanquake@gmail.com>

src/node/ui_interface.cpp

@@ -6,14 +6,14 @@
 
 #include <util/translation.h>
 
-#include <boost/signals2/last_value.hpp>
+#include <boost/signals2/optional_last_value.hpp>
 #include <boost/signals2/signal.hpp>
 
 CClientUIInterface uiInterface;
 
 struct UISignals {
-    boost::signals2::signal<CClientUIInterface::ThreadSafeMessageBoxSig, boost::signals2::last_value<bool>> ThreadSafeMessageBox;
-    boost::signals2::signal<CClientUIInterface::ThreadSafeQuestionSig, boost::signals2::last_value<bool>> ThreadSafeQuestion;
+    boost::signals2::signal<CClientUIInterface::ThreadSafeMessageBoxSig, boost::signals2::optional_last_value<bool>> ThreadSafeMessageBox;
+    boost::signals2::signal<CClientUIInterface::ThreadSafeQuestionSig, boost::signals2::optional_last_value<bool>> ThreadSafeQuestion;
     boost::signals2::signal<CClientUIInterface::InitMessageSig> InitMessage;
     boost::signals2::signal<CClientUIInterface::NotifyNumConnectionsChangedSig> NotifyNumConnectionsChanged;
     boost::signals2::signal<CClientUIInterface::NotifyNetworkActiveChangedSig> NotifyNetworkActiveChanged;
@@ -42,8 +42,8 @@ ADD_SIGNALS_IMPL_WRAPPER(NotifyBlockTip);
 ADD_SIGNALS_IMPL_WRAPPER(NotifyHeaderTip);
 ADD_SIGNALS_IMPL_WRAPPER(BannedListChanged);
 
-bool CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, caption, style); }
-bool CClientUIInterface::ThreadSafeQuestion(const bilingual_str& message, const std::string& non_interactive_message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeQuestion(message, non_interactive_message, caption, style); }
+bool CClientUIInterface::ThreadSafeMessageBox(const bilingual_str& message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeMessageBox(message, caption, style).value_or(false);}
+bool CClientUIInterface::ThreadSafeQuestion(const bilingual_str& message, const std::string& non_interactive_message, const std::string& caption, unsigned int style) { return g_ui_signals.ThreadSafeQuestion(message, non_interactive_message, caption, style).value_or(false);}
 void CClientUIInterface::InitMessage(const std::string& message) { return g_ui_signals.InitMessage(message); }
 void CClientUIInterface::NotifyNumConnectionsChanged(int newNumConnections) { return g_ui_signals.NotifyNumConnectionsChanged(newNumConnections); }
 void CClientUIInterface::NotifyNetworkActiveChanged(bool networkActive) { return g_ui_signals.NotifyNetworkActiveChanged(networkActive); }

test/lint/lint-includes.sh

@@ -64,7 +64,7 @@ EXPECTED_BOOST_INCLUDES=(
     boost/preprocessor/cat.hpp
     boost/preprocessor/stringize.hpp
     boost/signals2/connection.hpp
-    boost/signals2/last_value.hpp
+    boost/signals2/optional_last_value.hpp
     boost/signals2/signal.hpp
     boost/test/unit_test.hpp
     boost/thread/condition_variable.hpp