Use GTK4's "suspended" state to stop updating the GUI graphs, vumeters, etc. when window is not visible

[nekohayo]

From looking at https://github.com/search?q=repo%3Aryonakano%2Freco%20suspended&type=code it does not seem like you use GTK4's "suspended" state.

There is great power saving potential by connecting to GTK4's "suspended" state signal and freezing everything non-essential when the user can't actually see the window.

Less CPU/GPU usage, less heat/noise, more battery life, etc.

Same issue as nokyan/resources#480, which has more details/explanations and examples.

[nekohayo]

Maybe the measurements of loudness would be inevitable (and maybe zero-cost because you're already doing the recording anyway) so that the graphs can be completed "without gaps" when they unfreeze, but I presume at least drawing the graphs could be suspended/paused... assuming that drawing those realtime graphs costs anything at all, of course.