Function moras::menu::file::exit_handler

fn exit_handler(event: &WindowMenuEvent)

Iterate all tabs, check if each tab is dirty, if so, display a dialog to ask user to save the file or not. If user choose to save, save the file and then close the tab. If user choose not to save, close the tab directly.

This function will emit a front_close_tab event to the window, and may emit multiple times if there are multiple dirty tabs need to be closed.