Keep sliders centred in the designer and running desktop apps

Slider tracks and thumbs can appear off-centre inside their control bounds, making it difficult to align sliders with neighbouring controls. Horizontal sliders should remain vertically centred, and vertical sliders should remain horizontally centred, in both the visual designer and running desktop applications.

Correct the layout for different control sizes and all tick-mark placements. Tick marks should remain correctly positioned and usable without shifting the track away from the control's centre. The designer preview and runtime must match.