applet: Support RTL layouts in volume slider

ivan tkachenko requested to merge work/ratijas/slope into master

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")

image(Muted when visually high)
Edited by ivan tkachenko

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