[1.0.0] Bugfixing, testing and documentation

Bugfixing:

• ArrayIndexOutOfBoundsException: Setting the itemState from a pack that has empty states throws this exception.
• Add null to all simple signal states
• If you try to place a signal on another signal, it opens the prioritization menu instead.

Testing:

• WILL BE ADDED LATER New v2 contentpacks [In progress: Sign and Simple Signal is done, Asset and Complexsignal missing)
• Existing v2 contentpacks (migrated from v1) [In progress: hvkompakt pack for v2]

Documentation:

• Remove LegacyMode (it vanished)
• Add signal prioritization
• Add simple signal to contentpack documentation
• Add migration documentation for v1 to v2 packs