Base: Remove Boost-based signals and switch to FastSignals.

src/App/PropertyExpressionEngine.cpp

@@ -91,7 +91,7 @@ struct PropertyExpressionEngine::Private
 {
     // For some reason, MSVC has trouble with vector of scoped_connection if
     // defined in header, hence the private structure here.
-    std::vector<boost::signals2::scoped_connection> conns;
+    std::vector<fastsignals::scoped_connection> conns;
     std::unordered_map<std::string, std::vector<ObjectIdentifier>> propMap;
 };