Add ItemsAdder support and general code improvements

[K0bus]

This pull request introduces support for ItemsAdder integration and includes various code improvements:

• Added ItemsAdder as a project dependency
• Implemented a parser for ItemsAdder items
• Checked item icons only if they are set in the configuration
• Replaced hardcoded strings with configuration getters
• Removed redundant JavaDoc on void methods
• Simplified conditional statements
• Removed an unnecessary aggregator
• Marked variables as final where appropriate

These changes enhance both functionality and code quality, ensuring better maintainability and future extensibility.

[K0bus]

Added support for multiple slot assignment in menu objects.
It's now possible to define a range of slots using the "start-end" format (e.g., "1-3" will assign the object to slots 1, 2, and 3).
This addition does not alter the original behavior: specifying a single slot still works as before.