Fix volume popup on mobile for RTL environment

Fix volume popup on mobile for RTL environment

First, it was using wrong expression for "muted" state icon. Second, popup wasn't reversing position around its parent, and it would clip out of bounds without any (non-negative) margins.

MobileVolumeButton: Fix popup sizing

Hard-coded height was not even enough for slider, thus squeezing it vertically, causing qqc2-desktop-style to render onto smaller texture, effectively only painting part of the control. After recent refactoring in qqc2-d-s, Sliders know their implicit size better than we do, so let's delegate sizing to them.

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How to test mobile: export QT_QUICK_CONTROLS_MOBILE=1

How to test RTL: export LANGUAGE=ar:en_US LANG=ar_EG.UTF-8 LC_ALL=

Before:

After: