Fix disabling the spinner animation when Busy indicator has no opacity

Summary:
Unlike the visible property, the opacity does not inherit parent values
and refers to the relative opacity of that item.

The current style code is checking that an item we can't control
externally has an opacity > 0, this doesn't really acheive anything.

This was a breakage from when we ported *to* QtQuickControls 1.

Test Plan:
I was running a plasmoid I had written years ago (my RSS tutorial)
and had abnormally high CPU usage in an animation that I didn't have then.

Code was:

BusyIndicator {
opacity: 0
running: true
}

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4729