feature: Add pin/unpin feature for side and bottom panels

[Ralalu]

Overview

This pull request introduces a pin/unpin feature for the side (left/right) and bottom panels in the FlexLayout library. It allows users to keep panels permanently visible ("pinned") or collapse them automatically ("unpinned"), similar to behavior in modern IDEs and dashboard layouts.

Motivation

• Improved flexibility: Users can control whether certain panels remain open or auto-hide.
• Better usability: Helps streamline workflows and reduce visual clutter, especially in complex layouts.
• Feature enhancement: Extends existing FlexLayout capabilities (docking, maximizing, popout) with a commonly requested interaction.

Screenshot