Improved GUI and other enhancements!

New:

• Main categories in the GUI can be moved around by changing their order in gui.yml!
• New ReceiverChatMessages option in config.yml to stop displaying chat messages to players who receive achievements.
• New list-rewards parameter in lang.yml. Same as list-reward, but for achievements with more than one reward!

Improvements:

• Progress bars in /aach list now all have the same width. This makes the GUI look more consistent and render slightly faster!
• Various small updates and code enhancements.

Bug fix:

• Fixed /aach add command when using it with the PlayedTime achievement category - hopefully for good this time!

Translations:

• Slovak translation, by KondzioSvk! Thanks for his contribution!