Several improvements to the print job queue

xs/xsp/GCodeSender.xsp

@@ -18,6 +18,7 @@
     void send(std::string s, bool priority = false);
     void pause_queue();
     void resume_queue();
+    void purge_queue();
     std::vector<std::string> purge_log();
     std::string getT();
     std::string getB();