GUI: Reduce CPU usage II - have smoother UX with faster refresh rate

[goneng]

• Set thread_sleep_interval to 0.01s for better UI responsiveness
• Improve thread waiting with thread.join() where applicable (thanks to @sbytnar)

[goneng]

Guys, please review when you have a minute -
I went with a simplified version of the fix, to keep the code clear - as suggested:

1. Shorter sleep-interval for higher refresh-rate - thanks @khronokernel
2. Replace time.sleep(...) with thread.join(...) where applicable - thanks @sbytnar

[khronokernel]

Sorry for the delay, got too caught up with work. Looks great to me, thank you again!