Floating Island World Generation

Minor changes:

• Cacti no longer spawns next to other blocks or other cacti
  
• Cacti now varies in height
  
• Server OPs are told about a new version when they join the server
  
• Server OPs do not get told about a new version in an update check, unless it's different to the last one (console still gets told)
  
• Trees spawn inside other trees a lot less now (still slightly common with redwood)
  
• Increased tree coverage on islands that have trees, to balance tree checks
  
• Valid spawn islands now check the island has at least 1 tree, instead of just being a certain type

Using a parent generator is subject to bugs out of my control, such as the parent generator affecting the placed islands or crashes because of incompatibility. Known issues:
Fjord trees occasionally generate on the islands.
Server crashes when using 173generator (not sure if up to date).