Brought back the "Terrain Properties..." context menu

This completes f7f904c70ed550390594b1a0a2c5650f8aff071e and reverts
most of 434d1aff8a4f5a015bc018d527947514bc949c48.
JSON plugin: Added terrain properties to JSON export
Merge branch 'master' into guid_generation
guid generation added to tilesets