Slider: Fix up RTL mode

Re-binding LayoutMirroring on every control is a bad practice, because it essentially disregards parent hierarchy preference.

MouseArea should opt out of inheriting mirroring for its anchors, because control's padding which it binds to are not mirrored automatically.

	Left-to-Right	RTL
Before
After

Not targeting KF5, because inheriting un-mirroring is unfixably broken there anyway.