GUIBaseContext :: doBeforeFillData - Pre-event handler before filling data into control

The handler is a reaction on the DEV_Fill event, which is generated when a list or edit control is going to be filled with data from its data source. The handler should return one of the following values:

• -1 (not handled) - the event handler will be removed from the handler list and is not called anymore.
• 1 (refuse) - Requested action will not be executed.
• 0 (ok) - Action is permitted to be executed.

Refusing a fill data action is possible only in case of refilling a control. Otherwise, the refuse request will be ignored.