Explicitly set no timeout for notifications

The notification spec handles -1 and 0 as special values:
-1 -> representation for "undef"
 0 -> infinitely
 values above 0 represent the timeout in ms.

The notification messages should disappear again and with 0, this insn't
the case. Setting it to -1 lets the notification manager decide the
timeout and displays the notification like all others.

src/notifications/ManagerLinux.cpp

@@ -62,7 +62,7 @@ NotificationsManager::showNotification(const QString summary,
         argumentList << text;                                // body
         argumentList << (QStringList("default") << "reply"); // actions
         argumentList << hints;                               // hints
-        argumentList << (int)0;                              // timeout in ms
+        argumentList << (int)-1;                             // timeout in ms
 
         static QDBusInterface notifyApp("org.freedesktop.Notifications",
                                         "/org/freedesktop/Notifications",