Add skeleton rendering and GUI integration

Introduce a new skeleton utility and integrate skeleton overlays into the GUI. Adds dlclivegui/utils/skeleton.py (SkeletonModel, Skeleton, loaders, render status/codes) and a helper to load DLC config skeleton. Wire skeleton drawing into display.draw_pose and the main window: add UI checkbox, auto-enable/disable logic, model-based skeleton configuration, and safe handling when keypoint counts or shapes mismatch. Also add a QScrollArea for the controls dock to allow scrolling.

Remove unused demo code

Add skeleton UI controls and gradient coloring

Add UI and rendering support for configurable skeleton appearance: color mode (solid or keypoint-gradient) and line thickness. main_window.py: introduce skeleton color combo, thickness spinbox, handlers (_on_skeleton_style_changed, _sync_skeleton_controls_from_model), and wire these into model loading and drawing flow; enable/disable controls appropriately and preserve auto-disable behavior. gui/misc/color_dropdowns.py: add gradient swatch icon and helpers to create/populate/get/set a skeleton color combo (supports Gradient and solid BGR swatches). utils/display.py: add keypoint_colors_bgr(colormap, num_keypoints) to produce exact BGR colors from a Matplotlib colormap and remove direct skeleton coupling from draw_pose. utils/skeleton.py: ensure draw_many forwards style, color_override and keypoint_colors to per-pose draw calls and validates pose shapes/keypoint counts. Overall this enables gradient coloring of skeleton lines based on keypoint colormap and exposes user controls to tweak skeleton rendering.

Author: C-Achard