Jakob Petsovits accepted merge request !448: (cherry-pick) applets/brightness: More robustness for surviving plugin destruction at Plasma / PowerDevil

Cherry-pick MR for !440.

Follow-up tweaks to further improve crash resistance when thebrightness applet's plugin is torn down while updating its displays:

• The scope guard, executed at the end of a display update loop,will now also check for aliveness to avoid writing to a member.Thanks to Anthony Fieroni for catching this.
• The aliveness check for queryAndInsertDisplay() will now beperformed after each call in the loop, instead of once at theouter while loop. More safety also for multi-monitor setups.

I'm pretty confident that this puts us at 100% coverage ofaliveness checks after co_await.

(cherry picked from commit f501c908)

Co-authored-by: Jakob Petsovits jpetso@petsovits.com