Add action to allow modified state check

[LucienMP]

Alter the action "{action:'status', message: ... } so that if it contains no parameters ( for example "{action:'status'}" ) then it fires a new event that gives the current modified state. Event fired should be "{event:modified, modified:1 or 0}"

See: https://desk.draw.io/support/solutions/articles/16000042544

{action: 'status', message: '...', modified: bool} can be sent at any time to display a message in the status bar. The optional modified flag is used to update the modified state. Instead of a message, you can use messageKey to specify a resource key for the message.