[FEATURE] Manually trigger defpoll variable update

[cherrynoize]

Description of the requested feature

I would like to be able to trigger a defpoll variable update so that if a button showing output from a script run periodically gets clicked, it instantly updates.

Proposed configuration syntax

eww update defpoll_variable

to update,

eww update defpoll_variable=new_walue

to force a new value until next update.

Additional context

I don't want to run all queried scripts every second for obvious resource management reasons. And I'm fine with a 5 second delay in an indicator when updated externally, but when I click on it it feels really jerky and uncomfortable having to wait a few secs before I get any feedback. Still I want the variable to also update on its own (although possible delayed) when updated externally.