fix(MetroWindow): add GlowBrush dependency property (fixes #4564)

[JyslaFancy]

Fixes #4564 — GlowBrush invalid property on MetroWindow.

Problem

The MahApps.Metro documentation references a GlowBrush property on MetroWindow, but this property was never implemented. Users following the docs example get 'GlowBrush invalid property' at design time.

Root cause

MetroWindow inherits GlowColor (Color?) from ControlzEx.WindowChromeWindow, but no GlowBrush convenience property was exposed.

Fix

Added a GlowBrush dependency property (of type Brush) that wraps the inherited GlowColor:

• Setting GlowBrush to a SolidColorBrush extracts the Color and assigns it to GlowColor
• Setting GlowBrush to null clears GlowColor
• The getter returns the current GlowBrushProperty value

This matches the pattern used by other brush properties on MetroWindow (e.g. NonActiveBorderBrush).

[BinToss]

I believe this should be changed in ControlzEx, instead. The GlowBrush/GlowColor issue originated from ControlzEx, after all—¹not MahApps.Metro.

        /// <inheritdoc cref="GlowWindowBehavior.GlowColor"/>
        [Obsolete($"Use {nameof(GlowColor)}")]
        public Color? GlowBrush
        {
            get => this.GlowColor;
            set => this.GlowColor = value;
        }

Or throw NotImplementedException. I'd prefer that it cause build errors instead of build warnings and runtime errors, but I don't know how to do that. Maybe a custom analyzer that checks for invalid references to GlowBrush and suggests using refactoring to GlowColor.

P.S. The removal of GlowBrush is obviously a breaking change, but breaking changes in transitive dependencies are often overlooked when upgrading explicit dependencies. It's no wonder this became an issue.

¹key-combo'ed em-dash and superscript. They're not LLM-generated.