Fix Generics for LayoutManager

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Description

Implements generic type support for St.Widget to provide proper type checking for layout managers. This change allows TypeScript to correctly infer and check layout manager types when using St.Widget with different layout managers.

Related Issue

Fixes #145

@KSXGitHub Can you try to test and review this changes?

Type of Change

• New feature (non-breaking change which adds functionality)
• Type definition update

Validation

The changes demonstrate proper generic type usage with St.Widget.

Test Location: examples/st-15-layout-manager/main.ts

Test Case:

export class GridLayoutWidget extends St.Widget<Clutter.GridLayout> {
    static {
        GObject.registerClass({
            GTypeName: 'GridLayoutWidget',
        }, this);
    }
    constructor() {
        super({
            layout_manager: new Clutter.GridLayout()
        });
        // TypeScript now properly recognizes layout_manager methods
        this.layout_manager.attach(
            new St.Label({ text: 'Test' }),
            0, 0, 1, 1
        );
    }
}

Expected Behavior:

1. Type generator correctly adds generic support for St.Widget
2. TypeScript properly infers layout manager types
3. IDE provides correct type checking and autocompletion for layout manager methods

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• The implementation matches other generic type implementations (like Gtk)
• Added tests to verify generic type generation

[KSXGitHub]

What TypeScript definitions does this code generate?