applet: Support RTL layouts in volume slider
Extracted code into VolumeSlider component of its own, and synced it from a recent PC3.Slider rewrite which implemented better RTL support.
Removed double-property hack with ignoreValueChange backing flag,
because in modern Qt/QML Slider::moved signal should be used to react
to user-driven changes while preserving the binding on value
property. Such binding still needs to be reset after user interactions,
so there's a simple rebinding on release in onPressedChanged handler.
Timer is no longer needed, and slider still works just fine without it.
See also: frameworks/plasma-framework!585
| Before | After |
|---|---|
 (Overlapping when visually "low")  (Muted when visually high)
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