Introduce get_main_id() function for TBasePainter

Simple text id can be required by some external libs (like dabc.js or
go4.js). For such libs text id can be generated. 
Not really required in JSROOT itself

Author: linev

scripts/JSRootPainter.js

@@ -2273,6 +2273,20 @@
       return res;
    }
 
+   /** @summary Returns string with value of main element id attribute
+   *
+   * @desc if main element does not have id, it will be generated */
+   TBasePainter.prototype.get_main_id = function() {
+      var elem = this.select_main();
+      if (elem.empty()) return "";
+      var id = elem.attr("id");
+      if (!id) {
+         id = "jsroot_element_" + JSROOT.id_counter++;
+         elem.attr("id", id);
+      }
+      return id;
+   }
+
    /** @summary Returns layout kind
     * @private
     */