This event occurs 1-3 times depending on cell's editability. onEditCell event passes the following parameters:
-   stage - stage of editing (0-before start[can be canceled if returns false],1- the editor is opened,2- the editor is closed); 
-   rId - id of the row; 
-   cInd - index of the cell; 
-   nValue - new value (only for the stage 2); 
-   oValue - old value (only for the stage 2). 
          grid.attachEvent("onEditCell", function(stage,rId,cInd,nValue,oValue){});
During normal edit process event fires 3 times 
-  cell edition initiated ( stage = 0 ) 
-  cell editor activated and ready for input ( stage = 1 ) 
-  cell edition finished ( stage = 2 ) 
The 3rd call can cause different results based on returned value
-  {bool}true - confirm edit operation  
-  {bool}false - deny edit operation ( previous cell value will be restored )  
-  {string} or {number} - provided value will be used a new value of the cell instead of one entered by user 
Event commonly used for validation