WinFormPost: cleaned up the 'disableBtn' feature

The convenience util function Ext.ux.RapidApp.WinFormPost provides
'disableBtn' option which will prevent the submit button from being
clicked more than once which is useful when calling long-running
processes. It also now re-enables on *caught* exception, which is one
where the controller returns success: false. This currently must be done
maunally on the controller side; throwing simple exceptions will not
re-enable the button