applet: warn user on raising max volume

This feature can be actively dangerous to your ears or your device's speakers if you use it to raise the volume of sound being played at a normal level (ask me how I know…).

However removing it entirely would remove the ability to boost annoyingly quiet sounds, which is useful.

Boosting quiet sounds on demand only when safe would be amazing, but is not technically feasibly at this time.

So as a compromise between these constraints, let's warn the user about the risks.