Add a performance warning helper

This is something that came up in the profiling workshop and has been discussed a little bit before too.

A helper util of some sort, perhaps a bit similar to the DebugView we already have, would be very useful when designing something in QDS. It might be enough to just have a border around the View3D itself that flashes red if performance drops under / jumps over a specified FPS number / frame time / render time even momentarily.

This would work well as an early warning system for doing things "Ok, after I added this 50Mb texture, I'm getting red. Perhaps I should do something about it?"

Adding requirements:

• configurable (maybe with Q_ env settings)
• present only on debug-builds
• documentation how-to
  • install
  • run