implement visual bell

[luebking]

#1356

So here's a POC for the approach that does not require an (aligned) API update of qtermwidget.
Since the graphicseffect approach also means I get to do whatever I want, I want a bloodshot (like in FP shooters :))

The final implementation would probably use the red of the color scheme and also be configurable (if one prefers only an audible or no bell), for now this is RFC on the topic.

[luebking]

Withdrawing the draft to get some comments (not gonna add a config as long as the silence notification is pending or this one gets a clear go! ;))

1. Do we add a visual bell at all?
2. Is it preferable to implement it this way (in the client) or to extend the qtermwidget API to allow enabling the visual bell?
   (though iirc r/n that's mutually exclusive w/ the notification, ie. the user can have either a visual or an audible bell, not both like with the presented approach)
3. What's it gonna look like?
   The bloodshot is actually just semi-a-joke and ret-conned; I arrived at it because:
   • we are at liberty to implement the VB anyway we want
   • the inversion at night flashes a bright light into a dark room, so maybe something more muted
   • vignette/inverted vignette are too subtle/aggressively bright
   • what's a muted but alerting color that's not too bright and probably no sane beings terminal background?
   • obviously red!
   • at this point I realized that I had just re-invented the bloodshot and figured I'm just gonna own that.
   • in a final version I'd probably explore the themes red, rather than hardcoding one (mine)
     I would however also understand objections to both the effect as well the class name.

[tsujan]

My answers are my opinions; they aren't meant to be a guide.

1. If possible with a minimal code, why not?
2. In this way (as implicit in 1).
3. No opinion yet. I'll tell you later.

That being said, if it's completed, I think we should merge it only after the next release, in November.