Use arena class slugs for class selection.

This commit is a minor refactoring of the class selection functionality
plugin-wide. Instead of selecting classes based on the "lowercase name"
of a class, commands and listeners are now "slug aware", as it were.

The ArenaClass class now uses its slug instead of its "lowercase name"
for equality and hash codes.

The `/ma class` command now tab completes slugs, but it still supports
class names as they appear in the config-file when executing the command
itself. The same applies to the `/ma classchest` command.

The sign handling in ArenaListener slugifies sign text instead of just
lowercasing and stripping spaces.

changelog.md

@@ -24,6 +24,7 @@ These changes will (most likely) be included in the next version.
 - New command `/ma addreward <player> <thing>` can be used to add a reward to an arena player's reward list. This can be useful for hooking into the rewards system from scripts or other plugins.
 - The Root Target ability now uses potion effects (slowness, slow falling, and negative jump boost) instead of repeated teleports. This should make for a smoother root experience.
 - Permissions for arenas and classes are now based on "slugs". It is now possible to configure permissions for arenas and classes with multi-word names (including "My Items"). Check the Permissions page on the wiki for details.
+- Commands that resolve class names now consistently resolve and tab complete "slugs" instead of arbitrarily "squashed" names. This greatly improves support for multi-word names.
 - Using `spectate-on-death: true` no longer forces players out to their join location/exit warp before moving them to the spectator area. This should prevent "jumpy" behavior in multi-world setups.
 - Players should now properly respawn at the spectator area rather than at world spawn on servers with plugins that override respawn locations.
 - Config-files with missing `pet-items` nodes no longer errors. A missing `pet-items` node in `global-settings` is treated as empty, i.e. no pet items will be registered.