Add itemcase types 'showcase' & 'shop'

Add itemcase types 'showcase' & 'shop' that can be saved and loaded.
Add itemcase selection system

Add a selection system that allows players to select itemcases for commands.
Also make managers accesible through the main plugin class.
Add CommandExecutor and the command 'modify'

Add CommandExecutor for command 'ic' and add the sub-command 'modify' that allows
players to modify the type of an itemcase.