qt: Fix play / pause icon to reflect current state

[cold-brewed]

Summary

The main toolbar pause button always displays the pause icon, even when the system is paused. When paused, the button goes into the checked state (but remains a pause icon).

This PR changes it to:

• Switch back and forth between play and pause icon depending on pause / run state
• Update the tooltip based on the current state
• Make the pause action button uncheckable so it will never appear checked or recessed (previous behavior, not necessary with the icon change)

Checklist

• I have discussed this with core contributors already

References

N/A