Add restore glyph and support for restored window state in title bar

[ProGraMajster]

This PR adds support for a restore glyph in the title bar.

The title bar buttons now correctly reflect the window state:

• Maximize icon when the window is in normal state
• Restore icon when the window is maximized

[jpobst]

Nice change! I think the icon isn't quite correct, the "front" window should "hide" the one behind it:

Right now it's two interlocking rectangles:

[Copilot]

Pull request overview

Adds a “Restore” glyph and updates the title bar’s maximize/restore button to reflect the current FormWindowState, improving visual correctness when toggling between Normal and Maximized.

Changes:

• Introduces a Restore glyph option for title bar buttons and updates painting logic to render it.
• Adds ControlPaint.DrawRestoreGlyph and a small SKCanvas.DrawPath helper for stroking SKPaths.
• Updates FormTitleBar to switch maximize button glyph based on the current window state.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/Modern.Forms/FormTitleBar.cs	Adds restore glyph support, a mutable glyph property, and logic to update the maximize/restore icon based on window state.
src/Modern.Forms/Extensions/SkiaExtensions.cs	Adds a convenience DrawPath extension for stroking paths with a color/thickness.
src/Modern.Forms/ControlPaint.cs	Implements the restore glyph drawing routine using overlapping “window” outlines.

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