[Notifications] Check pause button when job is paused

To provide some more direct feedback that the job is paused and an easy to spot place to click to resume.

I chose to check the button instead of changing the icon like is done in Media Controller for more visual feedback. A job is also seldom paused so having it more obvious when it is the case aids the user better.

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There's two possible variants:

This patch implements the bottom variant. I think merely changing the icon gets lost between the progress bar and stop button. The top one is what Plasma in the old notification system did.

Also I think this would be fine for 5.20