Multiplayer, JSR 305, and Code Cleanup

Overview

• Synchronized menu interaction between all viewers
• @Redrield added JSR 305 annotations. (Thanks!)
• Added support for java.util.Optional in State
• Cleaned up the code quite a bit

Breaking Changes:

• Methods for getting the values in State have been changed to be more conventional:
  • current() -> getCurrent()
  • previous() -> getPrevious()
  • Doesn't exist -> getOptionalCurrent()
  • Doesn't exist -> getOptionalPrevious()

New Elements, Bug Fixes, and more

• Added HorizontalRuleElement, a straight horizontal line taking up the whole chat width
• Added GroupElement, a group of elements.
• Added element visibility
• Fixed width in BooleanElement
• Fixed width in IncrementalElement
• Reworked element interaction commands
• Made element overlap error more verbose
• Added method to remove elements

(Bug Fix) VerticalSelectorElement

• Fixed bug that prevented VerticalSelectorElement from working at all.
• A few minor additions