Add TrackBar control with renderer and demo panel

[ProGraMajster]

Added TrackBar control with renderer and demo panel.

Supports mouse, keyboard and wheel input.
Includes tick marks and vertical orientation.

[Copilot]

Pull request overview

Adds a new TrackBar (slider) control to Modern.Forms, including rendering/layout logic and a ControlGallery demo panel to showcase usage (orientation, ticks, snapping, input methods).

Changes:

• Introduces TrackBar control with keyboard/mouse/wheel interaction, snapping, and tick configuration.
• Adds TrackBarRenderer and registers it in RenderManager.
• Adds TickStyle enum and a ControlGallery TrackBarPanel, plus a navigation entry in the gallery MainForm.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/Modern.Forms/TrackBar.cs	New TrackBar control implementation (range/value, input handling, snapping).
src/Modern.Forms/TickStyle.cs	New enum for tick-mark placement options used by TrackBar/renderer.
src/Modern.Forms/Renderers/TrackBarRenderer.cs	New renderer for drawing the track/thumb/ticks and doing value↔position mapping.
src/Modern.Forms/Renderers/RenderManager.cs	Registers the TrackBar renderer so the control can paint.
samples/ControlGallery/Panels/TrackBarPanel.cs	Adds a demo panel showing multiple TrackBar configurations and interactive options.
samples/ControlGallery/MainForm.cs	Adds TrackBar to the gallery tree and panel switch logic.

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[jpobst]

The only issue I found is: the mouse wheel and PageUp/PageDown do not work reliably when using SnapToTicks = true.

I think if the trackbar is using SnapToTicks and the user uses the mouse wheel or keyboard we should move to the next tick, even if that's not what SmallChange would imply.

[jpobst]

Great work! Thanks!