[xswiftbus] Use std::string_view for performance in CDBusMessage

This avoids repeated dynamic memory allocations in dbusMessageHandler
methods and elsewhere due to repeatedly constructing temporary strings.

std::string_view is a C++17 feature but also part of the Library
Fundamentals TS and available in all our compilers.

src/xswiftbus/dbusmessage.h

@@ -12,10 +12,23 @@
 #include "dbus/dbus.h"
 #include <string>
 #include <vector>
+#if defined(_MSC_VER)
+#include <experimental/string>
+#elif defined(__clang__)
+#include <string_view>
+#else
+#include <experimental/string_view>
+#endif
 
 namespace XSwiftBus
 {
 
+    #if defined(_MSC_VER) || defined(__clang__)
+    using std::string_view;
+    #else
+    using std::experimental::string_view;
+    #endif
+
     //! DBus Message
     class CDBusMessage
     {
@@ -45,13 +58,13 @@ namespace XSwiftBus
         dbus_uint32_t getSerial() const;
 
         //! Get the called interface name
-        std::string getInterfaceName() const;
+        string_view getInterfaceName() const;
 
         //! Get the called object path
-        std::string getObjectPath() const;
+        string_view getObjectPath() const;
 
         //! Get the called method name
-        std::string getMethodName() const;
+        string_view getMethodName() const;
 
         //! Begin writing argument
         void beginArgumentWrite();