Add CSS theme engine

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Status

• Allows theming using .css syntax without using a WebView
• Applies CSS to an abstract DOM
• Translates the computed CSS styles to WPF resources
• Reads theme data from themes/custom.css
• Hot-reloads when .css file changes
• Supports key FancyWM overlay elements
• Supports key CSS properties (color, border-width etc)
• Allows users to change theme name (other than custom.css) - will be done separately

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/* FancyWM\themes\custom.css */
panel-bar-handle,
panel-bar-button {
    color: white;
    background-color: rgba(0, 0, 0, 0.3);
}

panel-bar {
    border-color: white;
    border-width: 1px;
    border-radius: 4px;
    background-image: linear-gradient(30deg,
            rgba(44, 83, 133, 0.3) 30%,
            rgba(32, 48, 152, 0.6) 30%,
            rgba(44, 83, 133, 0.3) 60%,
            rgba(32, 48, 152, 0.6) 60%);
}

---

/* FancyWM\themes\custom.css */
panel-bar-handle,
panel-bar-button {
    color: white;
    background-color: rgba(255, 255, 255, 0.3);
}

panel-bar-handle {
    background-color: transparent;
}

panel-bar {
    border-color: white;
    border-width: 1px;
    border-radius: 12px;
    filter: drop-shadow(0px 0px 8px white);
    background-image: linear-gradient(30deg,
            red 0%, blue 20%, green 40%, green 60%, orange 60%);
}