webgui: introduce TWebWindow::Run(double) method

It allows to run webwindow functionality in non-main thread.
When no connection exists - just do sleep for specified time.
Should be regularly called

gui/webdisplay/inc/ROOT/TWebWindow.hxx

@@ -210,6 +210,8 @@ public:
 
    void Sync();
 
+   void Run(double tm = 0.);
+
    bool Show(const std::string &where = "");
 
    bool CanSend(unsigned connid, bool direct = true);

gui/webdisplay/inc/ROOT/TWebWindowsManager.hxx

@@ -44,9 +44,9 @@ private:
    std::string fAddr;                    ///<!  HTTP address of the server
    std::mutex fMutex;                    ///<!  main mutex to protect
    int fMutexBooked{0};                  ///<!  flag indicating that mutex is booked for some long operation
-   std::thread::id  fBookedThrd;         ///<!  thread where mutex is booked, can be reused
+   std::thread::id fBookedThrd;          ///<!  thread where mutex is booked, can be reused
+   unsigned fIdCnt{0};                   ///<! counter for identifiers
    // std::list<std::shared_ptr<TWebWindow>> fDisplays;   ///<! list of existing displays (not used at the moment)
-   unsigned fIdCnt{0}; ///<! counter for identifiers
 
    bool CreateHttpServer(bool with_http = false);
 

gui/webdisplay/src/TWebWindow.cxx

@@ -773,3 +773,16 @@ int ROOT::Experimental::TWebWindow::WaitFor(WebWindowWaitFunc_t check, double ti
 {
    return fMgr->WaitFor(*this, check, timelimit);
 }
+
+/////////////////////////////////////////////////////////////////////////////////
+/// Run window functionality for specified time
+/// If no action can be performed - just sleep specified time
+
+void ROOT::Experimental::TWebWindow::Run(double tm)
+{
+   if (tm <= 0) {
+      Sync();
+   } else {
+      WaitFor([](double) { return 0; }, tm);
+   }
+}