Fixed serialization of player heads.
Added placeholders support for gui title, item name and lore.

Trade offers edit improved and added ability to create more than 18 offers.

Fixed deserialization error when loading data from older plugin versions.
Added support for 1.18.

Changes:

- code quality improved
- added more logging and debug mode
- fixed custom potion serialization
- added ability to configure command labels
- added permissions for categories and all commands
- added ability to open shop for other player
- added ability to add custom model data to category item

additional data saving when shutting down the server for security

Improved performance of category data saving.
The data are now saved in separate files in the "categories" folder.

Updating requires nothing from user, just upload new version and restart server.

Added the ability to open category with the command: /shop <category name>