systray: Make SHIFT+Mousewheel change the volume, not the song.

This fixes a bug that recently hit its 8 year anniversary. :(

The big issue was that there was no easy way to tell which keyboard
modifiers were set for the systray since Qt's event loop didn't actually
receive the keyboard events.

There's a separate function you can call, queryKeyboardModifiers, which
makes a specific query to the desktop, and works even if Qt event loop
didn't receive keyboard events. This fixes the bug here, and hopefully
for everyone else.

In fixing the bug I noticed that we can set out-of-bounds volumes due to
lack of checking, so I fixed that too.

BUG:250355
FIXED-IN:18.08.3