[Feature Request] Add callback for dynamically adding swaybar JSON property

[NobodyXu]

Is your feature request related to a problem? Please describe.

While most property for the block can be specified ahead of time, certain property like "urgent", has to be specified dynamically
and different users have different take on what is "urgent".

Also, users would like a warning to pop out to warn them of this urgent situations, so they should be able to provide their own
callback.

Describe the solution you'd like

• python callback
• dlopen

Describe alternatives you've considered

This feature can certainly be builtin into swaystatus, but then user will not be able to extend it as they like and swaystatus
would become much more complex, since it would also need to write UI code for implementing popup, which is best to be left
for the user to implement.