Emit when a new widget is created.
Instead of going to the backend with each file URI to ask whether it can handle the current file or not,
we have this map of extension and priority pairs that we populate at application startup.
The real advantage of this approach is the following: Phosphor cannot run async code when invoking
for the command handlers
so the menu item would be always visible for the user even if the file type cannot be handled eventually.
Hopefully, we could get rid of this hack once we have migrated the existing Phosphor code to React.
Retrieves all open widgets that have been opened by this handler.
all open widgets for this open handler.